How Two Boys Made Their Own Electrical Apparatus
by Thomas M. (Thomas Matthew) St. John
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Transcriber's Note

The punctuation and spelling from the original text have been preserved faithfully. Only obvious typographical errors have been corrected.




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EIGHTH EDITION Price, postpaid, $1.00

This book contains 141 pages, 125 illustrations, and directions for making 152 pieces of apparatus. Size, 5x7-1/2; red cloth.

CONTENTS: Chapter I. Cells and Batteries.—II. Battery Fluids and Solutions.—III. Miscellaneous Apparatus and Methods of Construction.—IV. Switches and Cut-Outs.—V. Binding-Posts and Connectors.—VI. Permanent Magnets.—VII. Magnetic Needles and Compasses.—VIII. Yokes and Armatures.—IX. Electro-Magnets.—X. Wire-Winding Apparatus.—XI. Induction Coils and Their Attachments.—XII. Contact Breakers and Current Interrupters.—XIII. Current Detectors and Galvanometers.—XIV. Telegraph Keys and Sounders.—XV. Electric Bells and Buzzers.—XVI. Commutators and Current Reversers.—XVII. Resistance Coils.—XVIII. Apparatus for Static Electricity.—XIX. Electric Motors.—XX. Odds and Ends.—XXI. Tools and Materials.

"The author of this book is a teacher and writer of great ingenuity, and we imagine that the effect of such a book as this falling into juvenile hands must be highly stimulating and beneficial. It is full of explicit details and instructions in regard to a great variety of apparatus, and the materials required are all within the compass of very modest pocket-money. Moreover, it is systematic and entirely without rhetorical frills, so that the student can go right along without being diverted from good helpful work that will lead him to build useful apparatus and make him understand what he is about. The drawings are plain and excellent. We heartily commend the book."—Electrical Engineer.

"Those who visited the electrical exhibition last May cannot have failed to notice on the south gallery a very interesting exhibit, consisting, as it did, of electrical apparatus made by boys. The various devices there shown, comprising electro-magnets, telegraph keys and sounders, resistance coils, etc., were turned out by boys following the instructions given in the book with the above title, which is unquestionably one of the most practical little works yet written that treat of similar subjects, for, with but a limited amount of mechanical knowledge, and by closely following the instructions given, almost any electrical device may be made at very small expense. That such a book fills a long-felt want may be inferred from the number of inquiries we are constantly receiving from persons desiring to make their own induction coils and other apparatus."—Electricity.

"At the electrical show in New York last May one of the most interesting exhibits was that of simple electrical apparatus made by the boys in one of the private schools in the city. This apparatus, made by boys of thirteen to fifteen years of age, was from designs by the author of this clever little book, and it was remarkable to see what an ingenious use had been made of old tin tomato-cans, cracker-boxes, bolts, screws, wire, and wood. With these simple materials telegraph instruments, coils, buzzers, current detectors, motors, switches, armatures, and an almost endless variety of apparatus were made. In his book Mr. St. John has given directions in simple language for making and using these devices, and has illustrated these directions with admirable diagrams and cuts. The little volume is unique, and will prove exceedingly helpful to those of our young readers who are fortunate enough to possess themselves of a copy. For schools where a course of elementary science is taught, no better text-book in the first steps in electricity is obtainable."—The Great Round World.

Exhibit of Experimental Electrical Apparatus


While only 40 pieces of simple apparatus were shown in this exhibit, it gave visitors something of an idea of what young boys can do if given proper designs.



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R51—HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS. A book containing complete directions for making many kinds of electrical apparatus. Seventh edition; 141 pages; cloth. Price $1.00

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Ask your Bookseller, Toy-dealer or Electrical Store for our Books, Games, Toys and Electrical Apparatus.

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THOMAS M. ST. JOHN, Cascade Ranch, East Windham, N.Y.

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Containing Complete Directions for Making All Kinds of Simple Apparatus for the Study of Elementary Electricity



Author of "Fun With Magnetism," "Fun With Electricity," "The Study of Elementary Electricity and Magnetism by Experiment," "Things A Boy Should Know About Electricity," etc.

Eighth Edition

Thomas M. St. John Cascade Ranch East Windham New York

Copyright, 1898, by Thomas M. St. John

How Two Boys Made Their Own Electrical Apparatus.



I. Cells and Batteries, 5

II. Battery Fluids and Solutions, 15

III. Miscellaneous Apparatus and Methods of Construction, 20

IV. Switches and Cut-Outs, 28

V. Binding-Posts and Connectors, 32

VI. Permanent Magnets, 37

VII. Magnetic Needles and Compasses, 40

VIII. Yokes and Armatures, 45

IX. Electro-Magnets, 51

X. Wire-Winding Apparatus, 60

XI. Induction Coils and Their Attachments, 64

XII. Contact Breakers and Current Interrupters, 75

XIII. Current Detectors and Galvanometers, 78

XIV. Telegraph Keys and Sounders, 92

XV. Electric Bells and Buzzers, 104

XVI. Commutators and Current Reversers, 110

XVII. Resistance Coils, 114

XVIII. Apparatus for Static Electricity, 117

XIX. Electric Motors, 122

XX. Odds and Ends, 133

XXI. Tools and Materials, 137-141


The author is well aware that the average boy has but few tools, and he has kept this fact constantly in mind. It is a very easy matter for a skilled mechanic to make, with proper tools, very fine-looking pieces of apparatus. It is not easy to make good apparatus with few tools and a limited amount of skill, unless you follow simple methods.

By following the methods given, any boy of average ability can make the apparatus herein described.

Most of the illustrations have been made directly from apparatus constructed by young boys.

It is impossible to describe the different pieces of apparatus in any special or logical order. It is taken for granted that you have some book of simple experiments and explanations to serve as a guide for the order, and to give you an idea of just the apparatus needed for the special experiments.

It would be foolish to start in and make all the apparatus described, without being able to intelligently use it in your experiments. Take up a systematic course of simple experiments, and make your own apparatus, as needed.

Before making any particular piece of apparatus, read what is said about the other pieces of the same general nature. This will often be a great help, and it may suggest improvements that you would like to have.

In case your apparatus does not work as expected, read the directions again, and see if you have followed them. Wrong connections, poor connections, short circuits, broken wire, etc., will make trouble. With a little patience and care you will be able to locate and correct any troubles that may come up in such simple apparatus.


How Two Boys Made Their Own Electrical Apparatus




1. Carbon-Zinc Cell. Fig. 1. If you have some rubber bands you can quickly make a cell out of rods of zinc and carbon. The rods are kept apart by putting a band, B, around each end of both rods. The bare wires are pinched under the upper bands. The whole is then bound together by means of the bands, A, and placed in a tumbler of fluid, as given in App. 15. This method does not make first-class connections between the wire and rods. (See Sec. 3.)


2. Carbon-Zinc Cell. Fig. 2. In case you want to make your cell out of carbon and zinc rods, and do not have any means of making holes for them in the wood, as in App. 3 and 4, you will find this method useful. Cut grooves, G, into one side of the wood, A, which should be about 4-1/2 x 1 x 1/2 in. The grooves should be quite deep, and so placed that the rods will be about 1/4 in. apart. A strip of tin, T, 1/2 in. wide, should be bent around each rod. The screw, S, put through the two thicknesses of tin will hold the rod in place. Another screw, X, acts as a binding-post. The zinc rod only is shown in Fig. 2. The carbon rod is arranged in the same way. Use the fluid of App. 15.

3. Note. When the bichromate solution of App. 15 is used for cells, the strong current is given, among other reasons, because the zinc is rapidly eaten up. This action goes on even when the circuit is broken, so always remove and wash the zinc as soon as you have finished.


4. Carbon-Zinc Cell. Fig. 3. The wooden cross-piece, A, is 4-1/2 x 1 x 1/2 in. The carbon and zinc rods, C and Z, are 4 in. long x 1/2 in. in diameter. The holes are bored, if you have a brace and bit, so that they are 3/4 in. apart, center to center. This makes the rods 1/4 in. apart. To make connections between the rods and outside wires, cut a shallow slot at the front side of each hole, so that you can put a narrow strip of tin or copper, B, in the hole by the side of each rod. Setscrews, S, screwed in the side of A, will hold the rods in place, and at the same time press the strips, B, against them. Connections can easily be made between wire and B by using a spring binding-post, D, or by fastening the wire direct to the strips, as shown in App. 4.

Use the battery fluid given in App. 15, and use a tumbler for the battery jar. This cell will run small, well-made motors, induction coils, etc. (See Sec. 3.)


5. Carbon-Zinc Cell. Fig. 4. The general construction of this cell is the same as that of App. 3. There are 2 carbons, C, each 4 x 1/2 in. The holes for these are bored in A 1-1/4 in. apart, center to center. The zinc rod, Z, is a regular battery zinc, 6 x 3/8 in., and has a binding-post, Y, of its own. The rods, C, are held in A, and connections are made as explained in App. 3.

The wire, X, is fastened direct to the strips, B, as shown. When ready to use this cell, be sure that the wire connecting the carbons does not touch Z. (Why?) The other wire is connected to Y. The wooden piece is 4-1/2 x 1 x 1/2 in. Use the battery fluid of App. 15 in a tumbler. This cell will run small motors, and is good for induction coils, etc. (See Sec. 3.)


6. Experimental Cell. Fig. 5. Cut a strip each of copper, C, and zinc, Z. (See list of materials.) They should be about 2 in. wide and 4 in. long. Punch a hole through each, one side of the center, for screws, E. The wooden cross-piece, A, should be 4-1/2 x 1 x 7/8 in. The battery-plates, or elements, should be screwed to this, taking care that the screws, E, do not touch each other. If the holes are made in the position shown in Fig. 5, the screws can be arranged some distance apart.

The wires leading from the cell may be fastened under the screws with copper burs, or spring binding-posts (App. 42) can be slipped on the top of the plates.

The solution to be used will depend upon what the cell is to do. For simple experiments use the dilute acid (App. 14). If for small motors, use the formula given in App. 15. The zinc should be well amalgamated. (App. 20.)


7. Experimental Cell. Fig. 6. In some experiments a comparison is made between cells with large plates and cells with small ones. This form will be convenient to use where narrow plates are desired. Those shown are 4 x 1/2 in. They are screwed to the cross-piece, which is 4-1/2 x 1 x 7/8 in. Do not let the screws touch each other. The wires are fastened under the screw-heads.


8. Experimental Two-fluid Cell. Fig. 7. This cell has a zinc strip, Z, and copper cylinder, C, for the "elements." The porous cup, P C, is fully described in App. 11. Z is 5 x 1 in., and should be well amalgamated (App. 20). (Study reasons for amalgamation.) A zinc rod, like that shown in Fig. 4, may be used instead of the strip. The copper cylinder, C, nearly surrounds P C, and is made from a piece of thin sheet-copper, 6 x 2 in. The narrow strip, or leader, A, is 5 x 1/2 in. To fasten it to C, punch two small holes in C and A, put short lengths of stout copper wire through the holes, and hammer them down so that they will act as rivets, R. C can be hung centrally in the tumbler by bending A as shown. Y and X are spring binding-posts (App. 42). The battery wires can be fastened directly to Z and A, as suggested in Fig. 4.

9. Setting up the Cell. Arrange as in Fig. 7, but remove Z from P C. Pour some of the acid solution of App. 14 into P C until it stands about 2-1/2 in. deep, and at once pour the copper solution of App. 16 in the tumbler, on the outside of P C, until it stands at the same height as the liquid in P C. As soon as the liquids have soaked into P C, you can put Z in place, when the cell will be ready for use. Remove and wash Z, when you have finished, and if you wish to use this cell occasionally, remove the liquids and wash P C thoroughly in water. When dry it will be as good as new. The acid rapidly acts upon Z, so it is better to remove Z if you wish to leave the experimenting even for a few minutes only.

Put a few crystals of copper sulphate (blue vitriol) in the tumbler under the copper, to keep the copper solution saturated. (See text-book for the chemical action in this two-fluid cell.)


10. Two-fluid Battery. Fig. 8. When two or more cells are joined together the combination is called a battery. Fig. 8 shows two experimental cells joined in series. (Study methods of joining cells.) For convenience, and to keep them from being easily overturned, a frame has been made for them. The base, B, is 8 x 4 x 7/8 in. To the back of this is nailed the upright board, A, 8 x 4-1/2 x 1/2 in. On the top of A are 3 binding-posts, 1, 2, 3, which consist of metal strips 1-1/4 x 1/2 in. At the lower ends are screws which are connected with the cells, as shown. Spring binders can be easily slipped on and off the upper ends of the strips, so that one or two cells can be used at will. Bent strips, C, are nailed to B, to hold the tumblers firmly in place. This framework is not necessary, of course, to the proper working of the battery, but with it you are much less liable to upset the cells.


11. Gravity Cell. Fig. 9. In the two-fluid cell of App. 7 the fluids were kept apart by the porous cup. The gravity cell is really a two-fluid cell in which the two liquids are kept separate by the joint action of the current and the force of gravity. This cell is used for telegraph lines and for other closed-circuit work.

12. Construction. The zinc and copper, Z and C, Fig. 9, can be purchased about as cheaply as you can make them. There are many forms of the zincs, the one shown being called the crow-foot shape. The copper may be star-shaped, or as shown. If you wish to make C, use thin sheet-copper. Brush copper, 1-3/4 in. wide, is excellent for the purpose. Use a piece 12 or 15 in. long, and fasten to one end of it a copper wire, W, which must be covered with paraffined paper, or with rubber or glass tubing, where it passes up through the zinc sulphate solution and near Z. The glass jar, J, may be made from a large glass bottle. (See index for battery jars.)

13. To Set Up the Cell. (A) Place C upon the bottom of J, with W in the position shown. (B) Put in enough copper sulphate crystals to cover the bottom of J, but do not try to entirely cover C. At the start 1/2 lb. will be enough. (C) Pour in clean water until J is half full. (D) In another vessel dissolve 1 or 2 oz. of zinc sulphate in enough water to complete filling, J. (E) Hang Z in place (Fig. 9). Z must never touch C. They should be about 3 in. apart. A wire is attached to Z by the screw, S, and the hole, H. (F) Pour the zinc sulphate solution into J until it is within an inch of the top. It should cover Z.

(G) Connect the wires leading from Z and C to your sounder and key. (See diagram.) The cell will be weak at first, and it may not be able to run your sounder. If this is the case, "short-circuit" it by allowing the current to run around and around through the sounder and key, the switch being closed. You may also "short-circuit" the cell by joining the two wires together. This will, in a few hours, make the dividing line between the blue and white quite distinct, when the cell will be stronger. If you have a short line only, the battery may be short-circuited through your sounder or other coils of wire for 5 or 6 hours a day, without working it too much. It may be necessary to draw off some of the clear zinc sulphate, replacing it with clear water, if the blue line gets too low. Add water occasionally to make up for evaporation.

14. Regulating. The two solutions are kept apart by gravity, as the copper sulphate is heavier than the zinc sulphate. The dividing line between the blue and white solutions is fairly clear when the battery works well, and it should be about half way between C and Z, or about at J, Fig. 9. Never allow the blue to get as high as Z, as this indicates that the cell is not worked enough. The dividing line can be lowered by allowing it to run a buzzer or bell for a few hours, or by simply short-circuiting it. If the blue gets much below J it indicates that you are working the cell too hard, or that you need more copper sulphate. The harder the cell works, the more zinc sulphate is formed, and the lower the dividing line becomes.

15. Gravity Batteries of two more cells are needed when used on telegraph lines. You will need 1 cell to each sounder; that is, for a short line in the house with two sounders, use 2 cells. If you use a few hundred feet of wire running to a friend's house, use 3 cells. They must be joined in series; that is, the copper of one to the zinc of the other. (See diagram of complete telegraph line.) Do not use ground connections for short lines and home-made sounders; use a return wire. Do not use different kinds of cells upon the same line.


16. Storage Battery. To show the principle of storage batteries it is only necessary to use two plates of lead dipped in the battery fluid of App. 14. The cell may be made as in App. 5, Fig. 5, the only difference being that both plates are of sheet-lead. It will be an advantage to make the plates rough by hammering against them a coarse file. (See explanations and experiments with this form of cell in text-book.)


17. Porous Cups for Two-fluid Cells. Fig. 10. Very good porous cups can be made from ordinary blotting-papers, the average ones measuring 9-1/2 x 4 in. White ones should be used, so that you will not be bothered with the color coming out. Soak the edge along one end of the blotter in paraffine (Index) for about 1/4 in. When this is cold, roll the blotter into the form of a cylinder that is a little over 1 in. inside diameter, and have the paraffined end on the outside. This will make 2 thicknesses of paper all around, and a little to spare. Rub a hot nail over the paraffine to melt it, and stick the end to the cylinder. By putting on a little more paraffine along the edge where the end laps over, a good solid cylinder can be made. The cylinder should be strengthened still more by dipping each end into melted paraffine for about 1/8 in. The dark stripes around the ends and down the front of the cylinder (Fig. 10) are to represent the paraffine. Cut out a bottom about 1/4 in. larger all around than the cylinder. This may be paraffined to make it stiff. It should be fastened to the cylinder with paraffine. Paraffine is not acted upon or softened by water or acid, as is the case with glue.


18. Porous Cups for Two-fluid Cells. Instead of the blotters of App. 11, you can use short lengths of mailing-tubes, which are used to protect pictures, etc., when sent by mail. If you find that the particular tube tends to unwind when soaked, you can use a little paraffine along the edges of the spiral, as suggested in App. 11. Bottoms can be made for the cups as before.


19. Porous Cups for Two-fluid Cells. Ordinary unglazed earthen flower-pots make good cups. The hole in the bottom should be closed with a cork, or by fastening a piece of pasteboard over the hole with paraffine. The pasteboard may be fastened to the under side of the bottom more easily than to the upper side.

20. Note. It is a good idea to soak the top edge of porous cups for about 1/4 in. in paraffine to keep the solutions from crawling up by capillary attraction. If the solutions constantly evaporate from the soaked tops of the cups, they not only waste but they get the whole thing covered with crystals.



21. Sulphuric Acid. THIS ACID MUST BE HANDLED WITH GREAT CARE, as it (the concentrated) is very strong, and will burn the hands, eat holes in clothing, carpets, etc.; it will even char wood. Do not let any of it drop anywhere accidentally. If you wish to pour concentrated acid into a bottle, place the bottle to be filled upon a plate, and wipe all drops of acid from the outside of it afterward. The concentrated acid should be kept in tightly-corked bottles, as it absorbs moisture from the air very rapidly. Ordinary corks should be paraffined if they are to be used in acid bottles, or they will be soon eaten up.

22. Mixing. When sulphuric acid and water are mixed, considerable heat is produced. Never pour water into the acid, as the heat would be produced so rapidly that the vessel containing the mixture might break. Always pour the acid into the water, and thoroughly stir the mixture at the same time. Earthen vessels do not break when heated as easily as glass ones. The mixing may be done in ordinary glass fruit-jars, if care be taken to pour the acid slowly into the water. The jars should be set in some larger dish, or in the sink, before adding the acid. If they get too hot, allow them to cool a little before proceeding with the mixing. As the acid is much heavier than water, it will immediately sink to the bottom of the jar, unless constantly stirred.

23. There are different grades of acid upon the market. For battery purposes you do not need the chemically pure (C P) acid. The ordinary "commercial acid" is all right, even though it is a little dark in color. You can get this at any drug-store. Get 5 or 10 cents' worth at a time.


24. Battery Fluid for Simple Cells. For the simple cell (App. 5), when it is to be used for experiments with detectors or in the study of polarization, etc., a very dilute acid is best. Mix 1 fluid ounce of commercial acid with 1 pint of water. This will make 17 fluid ounces (See App. 19), and your mixture will be one-seventeenth acid. Make up a pint or quart bottle of this at a time, and label it with the date:

Dilute sulphuric acid. 1 part acid, 16 parts water. Apparatus 14.

25. Note. Do not fail to paste a label on all bottles as soon as you have put anything into them. Give the date, contents, and any other information that will help you to reproduce the mixture again. Do not write on them any abbreviations or other things that you will soon forget.


26. Battery Fluid; Bichromate Solution. For running small motors, shocking coils, etc., this solution will be found good when used with the zinc and carbon elements given in App. 3 and 4. The bichromate destroys the hydrogen bubbles which help to polarize cells so rapidly when the plain dilute acid (App. 14) is used. (Study polarization.) The zinc used in this fluid must be well amalgamated (App. 20).

Directions. With 1 quart of cold water placed in a glass or earthen dish, slowly mix 4 fluid ounces of commercial sulphuric acid. Read Sec. 22 carefully. When this gets about cold, add 4 ounces of bichromate of potash. Powdered bichromate will dissolve more quickly than the lump. Keep this fluid in corked bottles, labelled, with date:

Bichromate Battery Fluid. Apparatus 15.

27. Always take the zinc from this fluid as soon as you have finished experimenting, or even if you have no use for the cell for a few minutes. The zinc and fluid are rapidly destroyed in bichromate cells even when the circuit is open. Always wash the carbon and zinc as soon as you take them from the fluid.


28. Battery Fluid. For 2-fluid cells (App. 7), a saturated solution of copper sulphate (blue vitriol) is needed. Place some of the crystals in a glass jar, with water, stir them around, and add the sulphate as long as it is dissolved. A few extra crystals should be left in the stock bottle so that the solution will always be saturated.


29. Vinegar Battery Fluid. For a few of the experiments with detectors, etc., good strong vinegar does well as the exciting fluid. This may be used with the copper and zinc or carbon and zinc elements. The amount of current given with vinegar and App. 4 or 5 is sufficient to show many of the simpler experiments.


30. Battery Fluid. Strong brine, made by dissolving ordinary salt in water, will produce quite a little current with App. 4 or 5. The presence of the current is easily shown with the astatic detectors.


31. Measures for Water, Acids, etc. If you do not own a graduated glass, such as druggists use for measuring liquids, the following plan will be found useful. In the mixing of battery fluids, etc., while it is not necessary to be absolutely exact, it is necessary to know approximately what you are doing.

An ordinary glass pint fruit jar may be taken as the standard. This holds 16 fluid ounces, or 2 ordinary teacupfuls. A teacupful may then be taken as 1/2 pint, or 8 fluid ounces. You can probably find a small bottle that will hold 1 or 2 oz., and you can easily tell how much it holds by filling it and counting the number of times it is contained in the pint can.

A slim bottle holding 1/2 pint can be made into a convenient measuring glass by scratching lines on it with the sharp edge of a hard file. The lines should be placed, of course, so that they will show how much liquid you must put into it to make 1 oz., 2 oz., etc. Instead of the file marks, a narrow strip of paper may be pasted upon the bottle, and the divisions shown by lines drawn upon the paper.


32. To Amalgamate Battery Plates. To keep the zinc plates or rods in cells from being eaten or dissolved when the circuit is opened, they should be amalgamated; that is, they should have a coating of mercury. The local currents (see text-book) aid in rapidly destroying the zinc, unless it is amalgamated. Do not amalgamate copper plates—merely the zinc ones.

33. Place a few drops of mercury in a butter dish. Dip the zinc into the solution of App. 14, then lay it upon a flat board. This is necessary with thin sheet-zinc, as it becomes very brittle when coated with mercury, and will not stand hard rubbing. If you also dip a very narrow piece of tin into the dilute sulphuric acid, you can use this as a spoon and lift one drop of mercury at a time from the butter dish to the zinc. By tapping the tin upon the zinc, the mercury will leave the tin. Put the mercury only where the zinc will be under the solutions in the cell, then rub the drops around with a small cloth that has been dipped in the acid. The zinc will become very bright and silvery, due to the mercury. Do not get too much mercury on it, just enough to give it a thin coat, as it will make the thin zinc so brittle that it will very easily break. Amalgamate both sides of the zinc.




34. For Annealing and Hardening Steel. (See text-book for reasons why some parts of electrical apparatus should be made of hard steel, while other parts should be made of soft iron.)

35. To anneal or soften spring steel so that you can bend it without breaking it, heat it in a candle, gas, or alcohol flame until it is red-hot; allow the steel to cool in the air slowly.

36. To harden steel, heat as before, then suddenly plunge the red-hot piece into cold water. This will make the steel very hard and brittle.

Small pieces may be held by pinching them between two pieces of wood. Needles and wires may be stuck in a cork, which will serve as a handle. (See text-book.)


37. Alcohol Lamp. Fig. 11. An alcohol lamp is very useful in many experiments, and it is better than a candle for annealing or hardening steel needles when making small magnets (App. 21). You can make a good lamp by using a small bottle with a wide opening. A vaseline bottle or even an ink bottle will do. Make a hole about 1/4 in. in diameter through the cork with a small round file, or burn it through with a hot nail. Make a cylinder of tin about 1-1/2 in. long and just large enough to push through the hole. The tin may be simply rolled up. If you have glass tubing, use a short length of that instead of the tin. For the wick, roll up some flannel cloth. This should not fit the inside of the tin tube too tightly. The alcohol should be put into the lamp when you want to use it, and that left should be put back into the supply-bottle when you have finished, as alcohol evaporates very rapidly. The flame of this lamp is light-blue in color, and very hot.

Caution. Do not have your supply-bottle of alcohol near the lamp when you light the latter, or near any other flame. The vapor of alcohol is explosive.


38. Spool Holder for Wire. Fig. 12. When winding magnets it is necessary to have the spool of wire so arranged that it will take care of itself and not interfere with the winding. If you have a brace and bit, bore a hole in a base 7/8 in. thick for a 1/4 in. dowel. The dowel should fit the hole tight. The spools of wire purchased can then be placed upon the dowel, where they will unwind evenly. The base may be nailed or clamped to a table.


39. Spool Holder for Wire. If you have no brace and bit to make App. 23, nail a spool to a wooden base, place a short length of dowel in the spool, and use this combination as a spool holder. Make the dowel fit the spool by winding paper around it.


40. To Make Holes in Wood. If you have a brace and a set of bits, or even a small hand-drill, it will be an easy matter to bore holes in wood. An awl should be used to make holes for screws, such as those used in making binding-posts, etc., as the wood is very liable to split if a screw is forced into it without a previously-made hole.

Red-hot nails, needles, etc., are easily made to burn holes of desired diameters. They may be heated in a gas flame or by means of the alcohol lamp (App. 22). Flat pieces of hot steel will burn narrow slots, and small, square holes may be made with hot nails.


41. To Make Holes in Sheet-Metal. Fig. 13. Holes may be punched in sheet-tin, copper, zinc, etc., in the following manner: Set a block of hard wood, W, on end; that is, place it so that you will pound directly against the end of the grain. Lay the metal, T, to be punched, upon this, and use a flat-ended punch. A sharp blow upon a good punch with a hammer will make a fairly clean hole; that is, it will cut out a piece of metal, and push it down into the wood. A sharp-pointed punch will merely push the metal aside, and leave a very ragged edge to the hole. A punch may be made of a nail by filing its end flat.


42. To Punch Holes through Thick Yokes, etc. As soon as 5 or 6 layers are to be punched at one operation, the process becomes a little more difficult than that given in App. 26. If you have an anvil, you can place the yoke over one of the round holes in it, and punch the tin right down into the hole, the ragged edges being afterward filed off. Hold the yoke as in App. 79 or 80 for filing. As you will probably have no anvil, lay an old nut from a bolt upon the end of the block of wood (App. 26), place the metal to be punched over the hole, and imagine that you have an anvil. Very good results may be obtained by this method. The size of nut used will depend upon the size of hole wanted.


43. To Straighten Wires. It is often necessary to have short lengths of wires straight, where they are to be made into bundles, etc. To straighten them, lay one or two at a time upon a perfectly flat surface, place a flat piece of board upon them, then roll them back and forth between the two. The upper board should be pressed down upon the wires while rolling them. If properly done, the wires can be quickly made as straight as needles.

44. Push-Buttons. Nearly every house has use for one or more push-buttons. The simple act of pressing your finger upon a movable button, or knob, may ring a bell a mile away, or do some other equally wonderful thing.


45. Push-Button. Fig. 14. This is made quickly, and may be easily fastened to the window or door-casing. One wire is joined to A and the other to C. B is a strip of tin or other metal, about 5/8 in. wide and 2 in. long. It is bent so that it will not touch A unless it is pressed down. This may be placed anywhere, in an electric-bell circuit or other open circuit, where it is desired to let the current pass for a moment only at a time.


46. Push-Button. Fig. 15 and Fig. 16. By placing App. 29 in a box, we can make something that looks a little more like a real push-button. Fig. 15 shows a plan with the box-cover removed, and Fig. 16 shows a view of the inside of it, a part of the box being cut away. C, Fig. 15, is a wooden pill-box 1 in. high and 1-3/4 in. in diameter. Make a 1/4 in. hole in the cover of C for the "button," G, which is a short piece of 1/4 in. dowel. This rests upon a single thickness of tin, D, which is cut into a strip 3/8 in. wide and about 1-1/4 in. long. In the bottom of C are two holes just large enough to allow the screws E and F to pass through. The wires, A and B, pass from the binding-posts, X and Y, through small holes burned through the sides of the box, and are fastened under the screw-heads. The whole box is screwed to the wooden base, which is 3 x 4 x 7/8 in., by the screws, E and F. D should have enough spring in it to raise itself and G when the pressure of the finger is removed. The circuit will be closed only when you press the button.


47. Push-Button. Figs. 17, 18, 19. Fig. 17 shows a top view or plan of the apparatus. Fig. 18 is a sectional view; that is, we suppose that the button has been cut into two parts along its length and through the center line. Fig. 19 is an enlarged detail drawing of the underside of the spool, C. The same part is marked by the same letter in all of the figures.

Saw an ordinary spool, C, into two parts. One-half of C will serve as the outside case for the button. The part to be pressed with the finger is a short length of 1/4 in. dowel. To keep this from falling out of the hole in C, a short piece of wire nail, N, has been put through a small hole in its lower end. A slot, F, has been burned or cut into the underside of C, so that N can pass up and down in it when D is raised and lowered. The rod, D, rests upon A, one of the contacts. This is a straight piece of tin, cut as shown in Fig. 17, the narrow part being 1/4 in. wide and 1-1/4 in. long. The wide part is 3/4 in. wide and 1 in. long. The other contact, B, is the same size as A. A deep groove, a little over 1/4 in. wide, is cut into the base so that the narrow part of B can be bent down below the end of A. The base shown is 4 x 2-1/2 x 7/8 in. The spool, C, is fastened to the base by 2 screws or wire nails put up through the base, their positions being shown by the dots at E, Fig. 17. X and Y, Fig. 18, are 2 screw binding-posts. It is evident that the current cannot pass from X to Y, unless the button, D, be pressed down so that the end of A will touch B.


48. Sifter for Iron Filings. Fig. 20. In making magnetic figures with iron filings, it is an advantage to have the particles of iron fairly small and uniform in size. A simple sifter may be made by pricking holes in the bottom of a pasteboard pill-box with a pin. The sifter may be put away with the filings in it, provided you turn it upside down.


49. Sifter for Iron Filings. Fig. 21. Punch small holes in the cover of a tin box with a small wire nail. If you have occasion to use sifters for other purposes, the different sizes can be made by using larger and smaller nails to punch the different tin covers. But one size of nail should be used for one sifter.


50. Sifters may be made by pricking holes in an envelope. A sifter with very small holes can be made of a piece of muslin cloth. This can be used in the form of a little bag, or a piece of it can be pasted over the open bottom of a pill-box.


51. To Cut Wires, Nails, etc. If you have no wire-cutters, or large shears, you can cut large or small wires by hammering them against the sharp edge of another hammer, an anvil, or a piece of iron. Do not let the hammer itself hit upon the edge of the anvil. The above process will make a V-shaped dent on one side of even large wires, or nails, when they may be broken by bending back and forth.



52. Switches, Cut-Outs. Where apparatus is to be used frequently, such as for telephone and telegraph lines, it pays to make your switches, etc., carefully. The use of these switches, etc., will be shown in the proper place. Their construction only will be given here.


53. Cut-Out. Fig. 22. Details. X, Y, and Z represent 3 binding-posts like App. 42. These are fastened to a wooden base that is about 3 x 5 x 3/4. The ends of the wires shown come from and go to the other pieces of apparatus. Q shows a stout wire or strip of 2 or 3 thicknesses of tin. Suppose we have an apparatus, as, for example, an electric bell, which we want to have ring when someone at a distance desires to call us. If we use a telephone or telegraph instrument we shall want to cut the bell out of the circuit as soon as we hear the call and are ready to talk. Suppose the current comes to us through the wire, A, Fig. 22. It can pass by the wire, C, through the bell and back to X. If we wanted simply to have the bell ring, the current could pass directly from X into the earth, or over a return wire back to the push-button at our friend's house. If, however, we are to use some other instrument, by lifting the end of Q out of X and pushing it into Y, the bell will be cut out, and the current can pass on wherever we need it.


54. Cut-Out. Fig. 23. The main features of this are like those of App. 36. The three binding-posts are like App. 46. Instead of a band of metal to change connections, as Q in App. 36, a stout copper wire is used. This can be easily changed from one of the upper binding-posts to the other, thereby throwing in or cutting out any piece of apparatus joined with the upper connectors.


55. Switch. Fig. 24. This simple switch has but one contact point, D, which is a screw-head. This switch may be used anywhere in the circuit by simply cutting the wire carrying the current, and joining the ends of the wire to the binding-posts X and Y. The metal strip, E, is made of 2 or 3 thicknesses of tin. It is 5/8 in. wide and about 5 in. long, and presses down upon D, when swung to the left, thus closing the circuit. The short metal strips shown are 5/8 x 1-1/4 in. The upper strip is joined to the end of E by a coiled copper wire, C W. (See App. 50.) If the current enters by the wire, A, it will pass through C W, E, D and out at B. The strip E is pivoted at F by a small screw. The base may be 3 or 4 x 5 x 7/8 in.


56. Switch. Fig. 25. By increasing the number of contact points and the wires leading from them, a switch may be made to throw in one or more pieces of apparatus. This variety of switch is useful in connection with resistance coils (Index). By joining the ends of the coils with the points 1, 2, 3, etc., more or less resistance can be easily thrown in by simply swinging the lever, E, around to the left or right. The uses of this will be again referred to.

Details. The base of the one shown in Fig. 25 is 4 x 5 x 7/8 in. thick. The switch, E, is a band of 2 thicknesses of tin 5/8 in. wide. It is pivoted at F with a screw. To the end of E is fastened a copper wire, which leads to the upper binding-post, X (App. 46). The apparatus has 5 contact points, marked 1, 2, 3, etc. These consist of brass screws and copper washers. With F as a center draw the arc of a circle that has a radius of 4 in. Place the screws 1, 2, etc., along this arc, and about 5/8 in. apart, center to center; that is, the screws are all 4 in. from F, and are, therefore, in the form of a curve.

The last screw forms a part of the binding-post, Y. Suppose 4 pieces of apparatus, marked A, B, C, and D, be connected with 1, 2, etc., as shown. These may be, for example, coils of wire to be used as resistance coils. If the current enters at X, it will pass along at E and be ready to leave at Y, as soon as E touches one of the contact points. If E be placed upon 1, the current will be obliged to pass through all of the coils, A, B, etc., before it can get to Y. In this case the resistance will be greatest. If E be now moved on to 2, only A will be cut out, and the total resistance reduced. By placing E upon 4, but one coil, D, will be in the circuit. When E is upon 5 the current will pass through the switch with practically no resistance. This is the principle upon which current regulators work. (Study resistance in text-book.) When E is in the position shown in Fig. 25 no current can pass.



57. Binding-Posts are used to make connections between two pieces of apparatus, between two or more wires, between a wire and any apparatus, etc., etc. They are used simply for convenience, so that the wires can be quickly fastened or unfastened to the apparatus. There are many ways of making them at home. The following forms will be found useful and practical. Although some that are given are really connectors instead of binding-posts, we shall give them the general name of binding-posts.


58. Binding-Post. About the simplest form is a screw, or a nail with a flat head. The bare wire may be placed under the head of the screw or nail before forcing it entirely into the wood. This will keep the end of the wire in place, and another wire may be joined electrically to the first by merely touching it to the screw-head, or by placing it under the screw-head.


59. Binding-Post. Fig. 26. This consists of a screw and a copper washer or "bur." The screw is a "round-headed brass" one, 5/8 in. long, number 5 or 7. The copper burs are No. 8, and fit nicely around the screws. By using 2 burs instead of 1, several wires may be easily joined together at one point. Scrape the covering from the ends of the wires, and place them between the burs.


60. Binding-Post. Fig. 27. A coiled spring serves very well as a connector. One end should be fastened to the apparatus, as shown, by clamping it under a screw-head. The other end of the coil should be pulled out a little, away from the other turns, so that you can stretch the spring in order to put the bare ends of wires between the turns. Any number of wires placed between these turns will be pinched and electrically connected. The coil should be about 1/2 in. long and less than 1/2 in. in diameter. You can make a coil by tightly wrapping stiff iron wire around a pencil. The steel wire springs taken from old window-shades are excellent for this purpose. They may be cut into lengths with tinner's shears.


61. Binding-Post. Fig. 28. Two copper or tin strips fastened at one end by a screw, the upper strip being bent a little at one end, make a connector that is useful for some purposes, where you want to make and break the connection frequently. The bare end of the wire which belongs to the apparatus is fastened under the screw-head. The outside wire, or wires, to be connected are pushed between the strips of metal. Another way is to fasten the outside wire to a strip of metal about 1/2 in. wide, and then push this between the strips shown in the figure. The strips shown should be about 3/4 in. wide and 1-1/4 in. long.


62. Binding-Post. Fig. 29. A combination made between App. 42 and 43 does well. Fasten a metal strip, 3/4 in. x 1-1/4 in., to the apparatus by means of a screw. The apparatus wire should be fastened under the screw-head. A short length of spring may be pushed upon the upright part of the strip, as shown. Into this you can quickly fasten the outside wires.


63. Binding-Post. Fig. 30. This makes a very simple and practical binding-post for home-made apparatus. It consists of a screw-eye, preferably of brass. The circle or eye should be about 3/8 or 1/2 in. in diameter. The thread on such a screw-eye will be about 1/2 in. long. Two copper burs are used to pinch the wires.


64. Binding-Post. Fig. 31. This consists of a screw, screw-eye, bur and a metal strip, 3/4 x 1-1/4 in. The apparatus wire should be fastened under the screw-head. Any outside wires which are to be joined to the apparatus should be clamped under the bur by turning the screw-eye. A small hole should be made in the wood before putting in the screw-eye. (See App. 25.) Do not turn the screw-eye too hard, or you will spoil the thread made in the wood.


65. Binding-Post. Fig. 32. The size of the bolt used in this form of binding-post will depend somewhat upon the thickness of the base of the apparatus. In general, a 3/4 or 7/8 in. base should be used where screws or screw-eyes are necessary. With this kind (Fig. 32) a thin base can be used. The head is shown counter-sunk into the bottom of the base. This is not necessary, provided at least 3 heads are placed far enough apart to form legs for the apparatus to stand on. Strips of wood may be nailed upon the underside of the base to make room for the heads in case they are not used as legs. The wires should be pinched between the nut and the copper bur shown. If the bolt is too large for a bur, an iron washer may be used. A washer may be made of tin, or two nuts may be used.


66. Binding-Post. Fig. 33. This is a suggestion for a combination of App. 44 and 47. It is useful in school apparatus. Wires may be permanently fastened on the right, under the nut, and a spring, as in App. 44, may be slipped on the metal strip at the left, which is held under the head of the bolt.


67. Mercury Connector. A cup of mercury may be used as a connector. Make a small hole about 1/4 in. in diameter and depth, in a piece of wood, and place 2 or 3 drops of mercury in this. The ends of wires dipped in this will be electrically connected.


68. Connector. Fig. 34. This shows how a wire may be fastened to one end of a short strip of tin. At the other end of the strip a slot is cut. This may straddle the body of a screw, or when left plain may be used to slip between the two metal strips shown in App. 43.


69. Binding-Post. Fig. 35. The ends of two or more wires may be quickly joined electrically by placing them between the nuts of a short bolt. By using 3 nuts the bolt will more easily connect a large number of wires.

Make Additional Notes and Sketches Here.



70. Permanent Magnets may be made in many ways and from many different kinds of steel. The steel used for needles, watch and clock springs, files, cutting tools, etc., is generally of good quality, and it is already hard enough to retain magnetism. (See Retentivity in text-book.)


71. Bar Magnet. A straight magnet is called a bar magnet. Magnetize a sewing-needle. For some experiments a needle-magnet, as we may call it, is better than a large magnet.


72. Bar Magnet. A harness-needle, which is thicker and stronger than a sewing-needle, makes an excellent bar magnet.


73. Bar Magnet. For long slim magnets use a knitting-needle. Some knitting-pins, as they are sometimes called, break off short when bent, but most of them will bend considerably before breaking. These slim magnets are excellent for the study of Consequent Poles. (See text-book.)


74. Flexible Bar Magnets. It is often necessary to have flexible magnets so that they may be bent into different shapes. These may be made from watch or clock springs, as such steel, called spring steel, will straighten out again as soon as the pressure is removed from it. Corset steels, dress steels, hack-saw blades, etc., make good thin flexible bar magnets.


75. Strong Bar Magnets may be made from flat files. The handle end may be broken off so that the two ends of the file shall be nearly alike in size. These should be magnetized upon an electro-magnet.


76. Compound Bar Magnets are made by first magnetizing several thin pieces of steel, and then riveting them together so that their like poles shall be together, and pull together. To make a small compound bar magnet, magnetize several harness-needles, or even sewing-needles, and then bind them into a little bundle with all the N poles at the same end. Melted paraffine dropped in between them will hold them together. Rubber bands may be used also, or, if but one end is to be experimented with, the points may be stuck into a cork, and the heads used to do the lifting.


77. Small Horseshoe Magnets may be made from needles or from other pieces of steel used for bar magnets. They should be annealed (App. 21) at their centers at least, so that you can bend them into the desired shape. In the case of bright needles, like harness-needles, the part annealed will become blackened. If you heat the center only, and the ends remain bright for about 1/2 inch, you will not need to harden the needle again. It is an advantage to have the center of the magnet a little soft, as it is not then liable to break. The ends alone may be hardened by holding the bent portion away from the candle or gas flame, while heating the ends. The bent steel should be magnetized by drawing its ends across the poles of a horseshoe magnet.


78. Flexible Horseshoe Magnets may be made of thin spring steel. The distance between the poles can be regulated at will by bending the steel more or less. The poles may be held at any desired distance apart by thread or wire, which should be wound around the legs of the magnet a little above the poles. This will keep the steel from straightening out.


79. Horseshoe Magnet. Fig. 36 and 37. Magnetize two harness-needles, and stick them into a cork so that the poles shall be arranged as shown. The distance between the poles can be regulated to suit. This forms a very simple and efficient magnet, with the advantages of a real horseshoe magnet.


80. Armatures. All home-made magnets should be provided with armatures, or keepers. These are made of soft iron on the regular magnets, and tend to keep the magnet strong. (See text-book.) For the bar magnets described, a piece of sheet-tin, upon which to lay them, is all that is needed for an armature. The lines of force will pass through this. For the horseshoe magnets described, strips of tin, soft iron wires, or even a wire nail placed across the poles will greatly aid in keeping in the strength. The little magnets should not be dropped or jarred. (Study the theory of magnetism in text-book.)



81. Magnetic Needles and Compasses consist chiefly of a short bar-magnet. When used to tell the directions, north, east, etc., the apparatus is generally called a compass. When we speak of the "needle," we really mean the compass-needle. The little magnet may be almost any piece of magnetized steel, provided it is arranged so that it can easily swing around. There are several ways of supporting the compass-needle. It may rest upon a pivot, it may be hung from a fine thread, or it may be floated upon water with the aid of a cork, etc.

82. Uses. We all know that compasses are used to point to the north and south, and we speak of the "points of the compass." This, of course, is the most important use of the compass, and it has been known for centuries. In the laboratory it is used to show or detect the presence of currents of electricity, and, in connection with coils of wire, it may show the relative strengths of two currents, etc. When used for such purposes it generally has special forms and sizes. (See Galvanometers and Detectors.)


83. Compass. An oily sewing-needle will float upon the surface of water, when it is carefully let down to the water. A little butter may be rubbed upon the previously-magnetized needle to make it float better.


84. Compass. Fig. 38 shows a magnetized sewing-needle floated upon a cork. The needle may be permanently fastened to the cork with a few drops of melted paraffine.


85. Compass. Fig. 39. With a sharp knife make a cut part way through a flat cork. Into the cut push a short length of magnetized watch-spring. In the illustration the spring is shown partly removed from the cut. Float the cork.


86. Compass. Fig. 40. Stick a pin, P, into a pasteboard, cork, or wooden base, B. Bend a piece of stiff paper double, as shown, and then stick through it, on each side, a magnetized sewing-needle, S N. The north poles of the needles should be at the same end of the paper. Why? Balance the paper upon the pin-pivot, and see it fly around to the north and south.


87. Compass. Fig. 41. It is an advantage to have a magnetic needle that is always ready for use. The support is made by driving a pin through the top of a wooden pill-box, which should be about 1-3/4 in. in diameter. This gives plenty of room under and around the needle. If the pin be left too long, it will not be possible to put the bottom and top of the box together when you want to put the compass away. Cut the pin off (App. 35) at the right length, so that the magnetic needle can be safely put away in the closed pill-box.

88. The "Needle," that is the short bar magnet, may be made of watch-spring. As the spring is already quite hard and brittle, it may be easily broken into desired lengths. It is always better to make 3 or 4 needles at a time, as some will swing more easily than others, and time will be saved in making them. Break off 3 or 4 pieces of thin spring, each about 1-1/2 in. long. Bend them as in Fig. 42. A good dent, not a hole, should be made at the center of each to keep them upon the support or pin-point. A "center punch," not too sharp, is the best tool to use, but a slight dent may be made with a sharp wire nail, provided the watch-spring is first annealed or softened. (See App. 21.) Do not place the spring directly upon iron or steel when making the dent, as these might injure the point of the punch, and the dent would not be deep enough. Fig. 42 shows a good way to make dents in steel springs. Place 2 or 3 layers of copper or lead between the anvil and the spring. A hammer or hatchet will do for the anvil. As the copper will give easily, a good dent may be made by striking the punch or nail with a hammer. If the spring has been annealed before denting it, it should be hardened again (App. 21) before magnetizing it, so that it will retain magnetism well. (See Residual Magnetism in text-book.)

89. Balancing. After a dent has been made, place the spring upon its support so that the pin-point shall be in the dent. It will, no doubt, need balancing. If one end is but slightly heavier than the other, the spring may be balanced by magnetizing it so that the lighter end shall become a north pole. This will then tend to "dip" and make the needle swing horizontally. If one end is much heavier than the other, it should first be magnetized and then balanced by cutting little pieces from the heavier end with tinners' shears, or by weighting the lighter end with thread, which may be wound around it. The finished compass-needle should swing very freely, and should finally come to rest in an N and S line after vibrating back and forth several times.


90. Glass-Covered Compass. A perspective view of this apparatus is shown in the tangent galvanometer. (See Index.) The outside band, E, is made of thick paper, 1 in. wide, and with such a diameter that it just fits around the glass. In this model, the glass from an old alarm-clock was used, it being 4 in. in diameter. Four pasteboard strips were sewed to the inside of the paper band E. They were made 7/8 in. long, so that the glass, when resting upon them, would be near the top of E.

The needle should be not over 1 in. long, if it is to be used in the galvanometer. A long slender paper pointer should be stuck to the top of the needle. Be careful to have the combined needle and pointer well balanced, so that it will swing freely. A circle graduated into 5-degree spaces should be fastened under the needle.

91. Astatic Needles. In the magnetic needles so far described, the pointing-power has been quite strong. By pointing-power we mean the tendency to swing around to the N and S. In App. 65 the 2 needle magnets had considerable pointing-power, because they helped each other. For some experiments in electricity a magnetic needle is required which has but little pointing-power; in fact, to detect the presence of very feeble currents by means of the needle, the less the pointing-power the better. Can you think of any way to arrange App. 65 so that it shall have very little pointing-power?


92. Astatic Needle. Fig. 43. Turn one of the needle magnets of App. 65 end for end, so that the N pole of one shall be at the same end of the paper as the S pole of the other. You can see that by this arrangement one needle pulls against the other. The magnetic field still remains about the little magnets, otherwise this combination would be of no value in the construction of galvanometers. The more nearly equal the magnets are in strength, the less the pointing-power of the combination.


93. Astatic Needle. Fig. 44. Magnetize two sewing-needles as equally as possible, by rubbing them over the pole of a magnet an equal number of times. Remove the covering from a piece of fine copper wire, say No. 30, and use the bare wire to wind about the needles, as shown. Be sure to place the poles of the little magnets as in the Fig. This combination may be supported by a fine thread. It is used for Astatic Detectors. (See Index.)



94. Yokes are used to fasten two straight electro-magnets together to form a horseshoe electro-magnet. The reasons for using them should be understood. Soft iron should be used for yokes and armatures, as this is the best conductor of lines of magnetic force. Sheet-tin is made of thin iron, which is coated with tin. (Try a magnet upon a tin can.) This soft iron is very easily handled, bent, and punched, and is very useful for many purposes. The tin from old tomato cans, cracker boxes, etc., is just as good as any. The method of making your yokes will depend entirely upon the tools at your command. Several ways are given. Y, Fig. 47, shows the position of the yoke.


95. Yoke. For the experimental magnets (App. 89) a fairly large yoke is required in order to have the magnets far enough apart. If you have only a nail punch (App. 26) with which to make holes in tin, you will be obliged to punch but one thickness at a time. (See method of punching sheet-metal, App. 26.) Cut 5 or 6 pieces of the tin, 3-1/4 x 1 in. With a center punch (tools) or sharp-pointed nail make small dents (2 in. apart) in each piece to mark the places where the holes are to be punched. Punch 5/16 in. holes in each piece. If you do this carefully, the holes in the different pieces will match, and the bolts can be pushed or screwed into these. When screwing in the bolt magnets turn them by their heads; do not pinch the coils, as this loosens the wire.

If you have a good punch, it is better to make the yoke as in App. 27, instead of using separate pieces of tin.


96. Yoke. Fig. 45 and 46. Cut a strip of tin 6 in. long by 3-1/4 in. wide. Bend one end of it so that it will lap over 3/4 in. (Fig. 46); hammer it down gently, then bend this over and over until the whole tin is used. The final result will be a flat roll, 3-1/4 by about 1 in. This should be hammered flat.

If you have the tools it is easy to drill two 5/16 holes in this strip. They should be 2 in. apart; that is, 2 in. from the center of one to that of the other. Start the holes with a center punch.

97. If you have no way of drilling the holes, they must be punched. (See App. 27.) This will make the strip bulge out on the underside around the holes. This bur, or most of it, should be filed off. (See App. 79 for method of filing thin pieces of metal.) The resulting yoke may be held firmly to the magnets by the use of 2 extra nuts, as in Fig. 67. Remember that the magnets must be held firmly in the yoke.


98. Yoke. The best way of making this, of course, is to cut a piece of bar-iron the right size. For 5/16 bolts the strip of wrought iron should be about 3/4 in. wide and 3/16 or 1/4 in. thick. Any blacksmith can make this and punch or drill the holes. If taps and dies (tools) are at hand, the hole may be drilled and tapped to fit the thread on the bolt. It is very easy to make good looking apparatus if you have, and can use, a whole machine shop full of tools. The lengths of yokes will depend upon the special uses to be made of them.


99. Yoke. Fig. 47. The yoke, Y, is a part of a carriage. This can be bought at a blacksmith's. The holes are already in, but it may require some filing before the nuts of the bolt magnets will fit down firmly.


100. Tin Armatures may be made by bending together 5 or 6 thicknesses of tin. Different forms of tin armatures are shown under telegraph sounders; these should have a hole punched at the center; through this is put a screw. The length of the armature will depend upon the distance the magnets are placed apart; they should be about 3/4 in. wide.


101. Nail Armatures. Fig. 48. A nail, N, placed through a piece of wood, A, will serve as a very simple armature. To make it a little heavier, if necessary, a piece of annealed iron wire, W, may be wound around N. Care should be taken to have the two parts fairly alike in size and weight.


102. Wire Armatures. Fig. 49. Annealed iron wires make good armatures. The short lengths of wire should be straightened (See App. 28) before binding them into a bundle. They may be held together with thread or paraffine, until they are in place, as, for example, in a wooden piece, A, Fig. 49. The bundle of wires should fit snugly into the hole made through A, and the wires should be bound together at each end with wire.


103. Trembling Armature. Fig. 50. Armatures to be used upon electric bells, automatic current interrupters, buzzers, etc., may be called trembling armatures. They may be made entirely of sheet-tin. The part, F, which gives it the spring, should be about 5/8 in. wide. Its length will depend upon the particular apparatus to be made. It is made of 2 thicknesses of thin tin. See Fig. 50 for dimensions. The part N projects beyond L. This may be used to tap against a regulating screw, or to fasten a hammer on for an electric bell. The part, L, should have about 4 layers of tin on each side of F, and it should pinch F tightly.


104. Trembling Armature. Fig. 51. When very rapid motions are desired in a trembling armature, App. 77 will be a little heavy. A light and quick-acting armature can be made of sheet-tin. The exact dimensions will depend upon the use to be made of it, but you will find the following a guide. Cut the part, B, E, out of thin tin. The covers and bottoms of tin cans are thinner than their bodies. The narrow part, B, should be about 1/4 in. wide and 2 in. long for a small apparatus, while E may be 3/4 in. square. Through E is a screw, which holds it firmly to a wooden piece, D, about 3/4 in. square. The part, E, can be made longer than its width, so that two screws can be used; this will keep A from jarring up or down.


105. To File Thin Metal Strips. Fig. 52. When sheet-metal is punched by the methods usually employed by boys, a bulge or bur is made on the underside around the hole. If this bur be hammered to flatten it, the hole is distorted and made smaller. It is better to file the bur down, at least part way. It is not convenient to file a piece of thin metal when it is held in a vise. It is better to use either a metal or a wooden clamp, as shown in Fig. 52; then the filing can be quickly and easily done. Y is the yoke to be filed. It is well to place a piece of metal, I, between the table and the end of the screw.


106. Clamp. Fig. 53. If you have no clamp to hold metal strips while filing them, you can put a screw, S, through one hole to hold the strip down fairly tight. Drive a nail, N, behind the strip. This will keep it from turning while you file the free end.

* * * * *

Electro-Magnetic Armatures. A description of this form of armature is given in the chapter on electric motors. (See Index.)



107. Electro-Magnets are absolutely necessary in the construction of most pieces of electrical apparatus. There are several ways of making them at home. To quickly make a good-looking one, a winder (App. 93) is required. We shall divide our electro-magnets into four parts: Core, washers, insulation, and coil.

Of course, you know that when a current of electricity passes through a wire, a magnetic field is produced around the wire. A coil of wire, or helix, has a stronger field than a straight wire carrying the same current, because each turn or convolution adds its field to that of the other turns. By having the center of the helix made of iron, instead of air, wood, or other non-magnetic bodies, the strength of the magnet is greatly increased. This central core may be fixed permanently in the coil, or be removable. For our purposes fixed cores are just as good as movable ones, and the coils are easily wound upon them.

When wire is wound by hand from a spool into a coil, or around a core, it soon becomes twisted and tangled. Make a winder. This will keep the wire straight and save much time.


108. Electro-Magnet. Fig. 54. Drive a nail into a board so that it will project about 3/4 of an inch. A soft, or wrought-iron, nail is best, but a short, thick wire-nail will do. If you do not have a thick nail, use an iron screw. Wind 3 or 4 layers of insulated copper wire around it, and fasten the bare ends of the wire down with bent pins. Number 24 wire will be found a good size for experimental purposes. Touch the wires leading from the battery to the ends of the coil, and see if the nail will lift pieces of iron.

109. Note. Always leave at least 6 in. of wire at the ends of all coils and windings. This is needed for connections and repairs, as the wire is liable to get broken at any time around the binding-posts.

110. Note. After you have wound wire upon a core or spool, keep it from untwisting by taking a loop or hitch around it with the wire. Fig. 55 shows how this is done. Pull the end of the wire enough to make the loop stay in place.


111. Electro-Magnet. Fig. 56. Cut annealed iron wire into pieces, 3 inches long, straighten them (App. 28), and tie them with thread into a bundle about 5/16 in. in diameter. Melted paraffine run in between the wires will hold them in together, but stout thread will do. Wind 3 or 5 layers of No. 24 insulated copper wire upon the soft iron core. This is useful for simple experiments, and this idea may be applied to magnets to be used in pieces of apparatus. Hold the bundle of wires in a vise, and file the ends smooth, before winding on the wire. Paraffine should be used to hold the turns of insulated wire together.


112. Electro-Magnet. Fig. 57. An electro-magnet with a removable core may be made by winding the wire on a spool. The core is made, as in App. 82, of soft iron wires, bound together with stout thread. A bolt may be used instead of the wire, but the wire loses its magnetism much quicker than a soft steel bolt would. (Study residual magnetism.) This magnet is strong enough for many purposes, but the wire is too far from the core, on account of the thickness of the wood, to make it efficient. The wire may be wound on by hand, but a winder (App. 93) will do much better and quicker work.


113. Horseshoe Electro-Magnet. Fig. 58. Bend soft iron wires, and make a bundle of them. If you wish to wind the wire around spools, the bundle cannot be very large. It will be found best to make the bundle about 3/8 in. in diameter, and not to use the spools. Strong paper should be wrapped once or twice around the legs of the horseshoe, and the insulated wire, say 4 layers, can then be wound directly upon this. (See Sec. 115 for method of making connection between the coils.) It is a little troublesome to wind wire upon a horseshoe like this, and for App. 85. Spools are handier, because each can be wound separately, and then be slipped in place. The ends of the horseshoe should be filed smooth.


114. Electro-Magnet. Fig. 59. An ordinary iron staple is useful as the core of a small magnet. One like this is shown also in Fig. 94, used as a telegraph sounder. It takes some time to wind 4 layers of wire on to each leg of the staple, so be sure to see Sec. 115 about the method of winding. In Fig. 59 the half-hitches (Sec. 110) are not shown. Coat the finished coils with paraffine.

115. Method of Joining Coils. Fig. 60. If A and B represent the two cores of a horseshoe electro-magnet, the coils must be joined in such a manner that the current will pass around them in opposite directions, in order to make them unlike poles. The current is supposed to pass around B, Fig. 60, in the direction taken by clock hands, while it passes around A in an anti-clockwise direction. The inside ends, Sec. 123, of the coils may be twisted together, or fastened under a screw-head. In Fig. 60 one coil is shown to be a continuation of the other.


116. Electro-Magnet. Fig. 61. Wind 6 layers of No. 24 or 25 insulated copper wire around a 5/16 machine-bolt that is 2-1/2 in. long. Fig. 61 shows one method of holding the bolt solidly in an upright position, so that magnetic figures can be easily made and the magnet studied. Two nuts are used, the lower one being counter-sunk, so that the base will stand flat upon the table. This bolt is shown without washers (Sec. 119), and will do fairly well to show the action of electro-magnets. The ends of the wire should always be left 5 or 6 in. long, and be led out to binding-posts. The coil may be held in place, and its turns kept from untwisting by coating it with paraffine. The base may be of any desired size.


117. Electro-Magnet Core. Fig. 62. This shows another method of fastening a bolt-core in an upright position. This is done without the use of two nuts. A strip of tin, T, 1 in. wide, is punched and slipped onto the 5/16 bolt before the nut is screwed on and the coil wound. This is fastened to the base by screws, S. Washers, W, are here shown. (See Sec. 119 for washers.)


118. Bolt Electro-Magnets are easy to make, according to the directions given, and they are, when finished, more like the regular purchased magnets than any of the other forms described. With proper batteries (App. 3, 4, etc.,) they can be used for a great variety of purposes, as will be seen. There are many forms of bolts in the market, but the ordinary "machine bolt," 5/16 in. in diameter, is best for our purposes. The ones 2 and 2-1/2 in. long are used.

119. Washers or coil ends are used on the bolt magnets so that considerable wire can be wound on closely and evenly. These are made out of thick pasteboard, which cuts smoother if it has been soaked in melted paraffine. Unless you know how, you will find it a hard job to make the hole in the exact center of the washer. The method of easily making washers is illustrated in Fig. 63.

First place a spool (the end of which is 7/8 or 1 in. in diameter) upon the table, and lay the pasteboard upon this. Push a large round nail through the pasteboard into the hole in the spool. The nail should be nearly as large as the hole. Use the large nail as a handle, and with the shears cut around the edge of the spool end. Cut the washer as round as possible, and be careful not to cut into the spool.

The holes in the washers will be a little smaller than the 5/16 bolt. This will make the washers hold tightly to the bolt when you force them on. Fig. 64 shows the bolt-core, with the washers in place. If you cannot get a large nail, a lead-pencil, or sharpened dowel, will do to force through the pasteboard.

120. Insulation of Cores. While the covering on the wire would probably be all that is necessary to thoroughly insulate the coil from the core, it is better to wind a layer or two of paraffine paper around the bolt (Fig. 65) before winding.

121. The Coils of wire to be used upon the bolt-cores should be put on with the winder (App. 93). For all ordinary purposes No. 24 or 25 single or double cotton covered copper wire will do. It is better to put on an even number of layers. The winding (See Fig. 70) begins at the nut-end of the bolt, and by using 6 or 8 layers of wire, instead of 5 or 7, both coil ends will be at the same end of the bolt.

122. Method of Winding the Coils. The winders used for bolt magnets are described in App. 91, etc. We shall suppose that the washer, W, Fig. 70, and the insulation, I, are upon the bolt before screwing it into the winder-nut, W N. Make a pinhole, P H, in the right-hand washer, as near the bolt-nut, B N, as possible. Stick about 6 in. of the wire through P H, and wind this end around W N, as shown, to hold the wire. The supply of wire should be upon a spool slipped onto some stationary rod (App. 23), so that you can give your entire attention to winding. Begin to turn the winder slowly at first. Turn the handle towards you when it is at the bottom, as in Fig. 70; that is, if you look at it from the side, turn the handle clockwise. Let the wire slip through your left hand as the turns are made, and guide it so that the turns will be close together. If they go on crooked, unwind at once, then rewind properly. You can guide the wire best by holding your left hand about 8 or 10 inches from the bolt. As soon as you reach the left side or head end of the bolt, feed the wire towards the right. If at any time the layers become rough on account of one turn slipping down between turns of the previous layer, fasten a piece of paraffine paper around the coil as soon as the imperfect layer is completed. Wind on 8 layers, and count the number of turns in one or two of them, so that you can tell about how many turns in all you have around the core. Make a "half-hitch" (see Sec. 110) with the wire when the last layer is finished, to keep it from unwinding, and leave a 6 in. end.

The coil should be protected by fastening around it a piece of dark-colored stiff paper. Paraffine paper is good for this purpose. With a little practice you will be able to rapidly and neatly wind on the wire. The winder-nut, W N, must hold the bolt solidly to keep it from wobbling.

123. We shall call the starting end of the wire which passes through P H, the inside end, and the end of the last layer the outside end. This can pass out between the washer and the paper covering.


124. Experimental Horseshoe Electro-Magnet. Fig. 66. Among the most useful pieces of apparatus for home use, is a good horseshoe electro-magnet. Fig. 66 shows a very convenient and practical form. With this, alone, can be shown all the principles of telegraph sounders, electric bells, etc. They are excellent for making magnetic figures (See text-book). You are supposed to be looking down on the App. in Fig. 66. The bolts are 2 in. apart center to center.

The bolt magnets are fully described in App. 88; the binding-posts, as App. 46; the yoke, as App. 71; the method of fastening to the base, as App. 90; the base is 5 x 4 x 7/8 in.; the magnets are made of 5/16 bolts, 2-1/2 in. long.

125. To Join the Coils, fasten the two inside ends (Sec.123) of the wire to a middle binding-post, and carry the outside ends to the two outside binding-posts. In this way you can use either magnet alone, if desired (See experiments in text-book), or change the polarity at will by changing the connections. (See Sec. 115 and 123.)


126. Fastenings for Electro-Magnet. Fig. 67. When both electro-magnets are to be permanently fastened to a base, especially if tin yokes are to be used, as in App. 89, it is best to use a nut on each side of the yoke. It is important to have a perfectly tight connection between bolt and yoke. Several ways of fastening the bolts and yokes are shown; but it will be found best to cut holes in the base for the lower nuts, and to screw the yoke directly to the base. This makes a solid and pleasing arrangement. For the experimental magnets (App. 89) make the yoke 3-1/4 in. long, and place the magnets 2 in. apart center to center.




127. Winder. Fig. 68. In case you do not have any means of making a smooth hole for the "bearings" of the winders of App. 93 and 94, you can use a spool for the purpose. B is the end of a piece of board about 1 in. thick, 3 in. wide, and 6 in. long. The spool, A, is laid upon this, a band of tin, T, being used to hold it down firmly upon the end of B. Screws, S, hold T down. A stove-bolt axle (See App. 93) is shown, and by using a nut, as explained, bolt magnets may be wound. By using the handle of App. 92, this arrangement can be used to wind almost anything, when used together with the attachment of App. 95.


128. Crank for Winders, etc. Fig. 69. This form of crank or handle will be found easier to make than the one in which a wire is expanded in the slot of a stove bolt, and it can be used for many purposes, especially where dowels serve as axles. Wrap a little paper around the end of the 1/4 in. dowel, D, and push it part way into the spool, A, then put in a set-screw, S, to keep A from twisting upon D. The straight end of the wire, H, should be put into a hole, B, and another set-screw used to fasten it into the spool.


129. Winder. Fig. 70. For winding bolt magnets, this form of winder is very useful. It consists of a "stove bolt," S B, 2 in. long (total length) and 5/16 in. in diameter.

130. Handle or Crank, H, is made of a stout wire, 4 in. long, bent at the lower end as shown. H is fastened into the slot of S B. To do this the end of H is hammered flat until it will just slip into the slot. It may be soldered there, or be made to fit by expanding it so that it will press out against the sides of the slot. To do this, place S B into a hole in an anvil, or hold it in a vise, being careful not to injure the thread. Place the flattened end of H in the slot, and strike it on top so that it will expand and be pinched in the slot; but do not pound it so hard that you split the bolt head. Three or four good center-punch dents upon the wire over the slot will help to expand it.

131. The Framework is made of wood, the dimensions being shown in Fig. 70. A 5/16 hole should be made for S B, the thread of which will stick through about 1/4 in. so that the winder-nut, W N, can be turned onto it. W N should be on but 2 or 3 threads of S B. This will leave part of it for the thread of the bolt magnet, and when this and S B meet in center of W N they will bind against each other and hold the bolt tight. The winder can be nailed or screwed at S to the edge of a table or held in a vise.


132. Winder. Fig. 71. This shows a winder that can be used for several purposes by arranging different attachments. It will be first described as shown in Fig. 71, where it is being used to wind a bolt magnet. The principal dimensions are shown in the figure. It is made of 3/4 in. wood about 3 in. wide, the two outer parts X and Z being nailed to the center one, Y, which is to be held in a vise, or fastened to the edge of a table. A 5/16 in. hole should be made through the upper part X and Z at one side of the center, so that a long 5/16 bolt can be put through and used as described in App. 93, if desired. A smaller hole, 1/4 in., should be made on the other side of the center for a 1/4 in. dowel. The dowel, D, is shown, and this size is a little smaller than the hole in ordinary spools, shown at A and B. One-quarter in. dowels can be made to fit fairly tight into the holes by wrapping paper around them. Five-sixteenth bolts can be screwed into the spool holes, shown by the bolt magnet in Fig. 71. To firmly hold a spool from twisting around upon the dowel-axle, a set-screw, S S, is needed. These are small screws, say 5/8 in. long, No. 5. A small hole should be made into the spool before forcing in the screw. (App. 25.)

The spools A and B are fastened in this way, by set-screws, to D. The handle, H, is made as in App. 93, in this case a short stove bolt, S B, being used and screwed into B. Fig. 69 shows a very simple form of handle for all such purposes, which may be used instead of the one here shown. The details of winding on the wire are given under App. 88.


133. Attachment for Winder. Fig. 72. By using this addition to App. 93 or 94, almost any ordinary kind of windings can be made. The wooden block, A, may be about 2 in. square and 7/8 in. thick. A set-screw, S, binds it to the dowel-axle, D, which is made to turn by one of the forms of cranks given, and which is held in one of the frameworks. Windings like that shown in App. 112, Fig. 85, can easily be done with this, the upright part, with the two spools, being screwed right to A of Fig. 72.



134. Induction Coils, or shocking coils, are rather expensive to buy, and altogether too complicated for boys to make by the methods usually given in books. The method here given is simple, the materials are cheap, and if you make them according to directions, you will have an apparatus that will, be able to make your friends dance to a rather lively tune. The amount of shock can be regulated perfectly (App. 103).

Winding. Full instructions have been given for making bolt magnets (App. 88). The winding of our induction coils is done in the same way by the same winder as the bolt magnets (App. 93), or by hand. You will find it a very tiresome and troublesome job, however, to wind on 12 or 15 hundred turns of fine wire by hand. Make a winder.

Several different forms of induction coils are shown. The coil is the most important feature, however, and we shall consider that separately. When you understand the construction of one coil, you can readily apply this to the different forms. Some form of contact breaker, or current interrupter, is needed also. These will be treated by themselves. The connections will be discussed under each form of apparatus.


135. Induction Coil; Construction of Coil Proper. Figs. 73, 74. An induction coil is a peculiar and wonderful apparatus. There are at least two coils to each one. These are both wound upon the same core. They are made of different sizes of wire, are wound separately, and the strangest thing of all is, that these two coils are not connected with each other in any way. If they were not thoroughly insulated from each other, the coil would be of no value. (Study induction.) The winding of the two coils is done as explained in App. 88.

136. The Core is made of a 5/16 machine bolt, 2-1/2 in. long. Leave but 2 or 3 threads at the end, just enough to fasten it solidly to the winder (App. 93). The washers should be about 1-5/8 in. apart inside, and they should be made around a spool (Sec. 119) that is fully 1 in. in diameter.

137. The Inside or Primary Coil could be wound directly upon the bolt; but it is much better to cover the bolt with one or two thicknesses of paraffined paper, I (Index), as shown. A pinhole, H, in the washer is for the inside end (see Sec. 123) of the primary coil, and the hole, J, is for the outside end of it.

The primary coil should be made of 3 layers of wire, which should be coarser than that used for the secondary coil. For our purposes it is best not to use a wire coarser than No. 20, and not finer than No. 24.

Use No. 24 insulated copper wire if you are going to connect ordinary batteries with it. A bichromate cell (App. 4) is best. Put about 6 in. (see Sec. 109) of wire through H, and with App. 93 wind on 3 layers of say No. 24 wire. There being an odd number of layers, the winding will stop at the head end of the bolt, where a half hitch (see Sec. 110) should be taken before passing the wire through the hole, J. Cut the wire 6 in. from the hole. Write down the number of turns of wire to each layer and the total number of turns. You now have a 3-layer coil, and a current passed through this will magnetize the bolt; you have—so far—merely an electro-magnet. Cover the primary coil with 2 layers of paraffined paper, K (Fig. 74), and put some paraffine between the edges of K and the washers, so that the wire of the secondary coil cannot possibly come in contact with that already wound on.

138. The Secondary Coil should be made of a large number of turns of fine wire. Do not use anything coarser than No. 30. This is a good size, as finer wire is very easily broken by unskilled hands. For the size of bolt mentioned put on 13 layers. There will be about 100 turns to each layer, making a total of about 1,300 turns of No. 30 wire. Write down the total number of turns in your coil. To start the secondary coil, make a pinhole, L, just outside of the insulation, K, of the primary coil. Put 6 in. of wire through this, wind the end around the nut (App. 93, Fig. 70), and wind on as evenly as possible 13 layers. If the layers become rough, it is well to put a band of paper around after each 3 or 4. When you have finished take a half hitch (Sec. 110), and leave a 6-in. length free. Cover the secondary coil with strong paper. This coil may be used on any of the forms of shockers given.


139. Induction Coil. Fig. 75. The base is made of a piece of board, 7 x 5 x 7/8 in. The locations of the different parts are shown in the figure. The coil is explained in detail in App. 96. It is fastened to the base by a thin copper strip, 4, which is bent over the coil and held down by screws, 3. If you haven't any copper you can use a narrow strip of tin. Do not use a wide piece of tin or iron. The coil may be held down firmly by strong twine placed around each end of it. The twine should pass through holes in the base, and be tied on the underside of the base. The binding-posts are like App. 46.

140. The Current Interrupter consists of a tin or copper strip, R, 6 in. long and 1/2 or 3/4 in. wide. At one end of R is a screw, S, which is used as a binding-post for the outside end, B, of the primary coil. (See Sec. 137.) Along the center line of the strip, R, are driven 1-in. wire nails, Q. These are placed 1/4 in. apart, and they should go into the wood enough only to make them solid. (See Fig. 81.) Do not drive them in so far that they will split the base. A stout wire, P, fastened at one end only completes the interrupter.

141. The Connections. The binding-posts, W and X, should be connected with the wires leading from a battery. Use the bichromate batteries of App. 3 or 4. A dry battery will do. If the current enters at X, it will pass around the primary coil (Sec. 137) and out through B into R. It can go no farther until the free end of P is made to touch R, or one of the nails, Q, when the circuit will be closed. The current will fly around and around through the battery, primary coil, and interrupter as long as the end of P touches a nail. The battery current does not get into the secondary coil at all. You can see, then, that the primary circuit, that is, the one passing through the coarse wire, will be rapidly opened and closed by bumping the free end of P along upon the row of nails.

The wires, C and D, coming from the secondary coil (Sec. 138) are in connection with Y and Z, to which are connected the wires leading from the handles (App. 101) held by the person receiving the shock.

142. To use the coil, arrange as explained. Let your friend hold the handles (App. 101) while you scrape the end of P back and forth along the row of nails. For those who cannot stand much of a shock, use a regulator (App. 103).


143. Induction Coil. Fig. 76. In case you wish to make the interrupter as a separate piece of apparatus, as App. 104, this arrangement will be found good. The base is 5 x 4 x 7/8 in. The coil is explained in App. 96, and the methods of holding it to the base are given in App. 97. The binding-posts are like App. 46.

The Connections. We shall suppose that you have the interrupter of App. 104, Fig. 81. The ends of the primary coil (Sec. 137) are fastened under the screws of X and W, and those of the secondary coil to Y and Z. Connect one battery wire with X and the other battery wire to the interrupter at S, Fig. 81. Fasten the end of a stout wire to W, and leave the other end free to scrape along on the nails, Q, of the interrupter. This will then open and close the primary circuit. The handles (App. 101) are connected with Y and Z, as explained in App. 97. Use the battery of App. 3 or 4.


144. Induction Coil. Fig. 77. If you wish to fasten your coil in an upright position the apparatus will look like Fig. 77. The base may be 5 x 4 x 7/8 in. The binding-posts are like App. 46. The coil is made as explained in App. 96; but to have all the ends of the coils come out at the bottom, as shown, an even number of layers of wire will be necessary. It will be just as well to have an odd number of layers as before, and to bring the wire ends down the side of the coil. The coil is fastened to the base with screws, S, passing through a tin strip, T, which has a hole punched for the bolt. T is squeezed between the regular nut on the bolt and an extra one on the underside of it. See Fig. 61 for suggestion of another method of holding bolts upright. The connections should be made with an outside interrupter, battery, and handles, as explained in App. 98.


145. Induction Coil. Fig. 78, 78-A, 78-B. In case you wish to make a larger coil than those already described, the following will be found practical. It is made in the same general way as before, an automatic interrupter, however, being added.

The Core is a machine-bolt, 4-1/2 in. long and 5/16 in. in diameter. You may use a carriage-bolt of the same dimensions, if you file away the square shoulder at the head end, so that it will be the same size as the body of the bolt. Paste a piece of thick paper upon the head, so that A will strike the paper instead of the iron. The Washers should be made around a spool that is fully 1 in. in diameter. (See Sec. 119.) The core should be insulated with paraffine paper before winding on the primary coil. (See App. 88.) The washers are 3-7/8 in. apart, inside. The winding of the coils should be done with App. 93, or some other winder. The winder-nut, W N, Fig. 70, must hold the long core perfectly tight, to avoid wobbling. The base is 8 x 5 x 7/8 in. The different parts are placed as shown. The coil is fastened to the base as in App. 97. For binding-posts see App. 46.

146. The Primary Coil (Sec. 137) is made by winding 3 layers of No. 24 insulated copper wire upon the insulated core. One end, 6, is fastened to W (See Sec. 109), and the other end, 5, is held under the screw-head, R. Wind at least two layers of paraffined paper around this coil before winding on the secondary coil.

147. The Secondary Coil (Sec. 138) is made of No. 30 insulated copper wire, there being 11 or 13 layers, each having about 200 turns. This makes, in all, about 2,500 turns of fine wire. If your winder works properly and the long core is strongly held by the winder-nut, you will have no trouble, although it takes a little time to wind on so many turns. The ends of this coil, 7 and 8, are fastened to Y and Z, which are made like App. 46. It will be found best to wrap a piece of thin paper around the coil after every 3 or 4 layers are wound on. This makes better insulation, and makes the winding easier. Protect the coil by covering it with thick paper. The whole coil, when completed, is about 1 in. in diameter.

148. The Automatic Interrupter (Figs. 78, 78-A, 78-B) consists of several parts. B, E, C is a piece of thin tin, all in one piece. The part, B, is 1/4 in. wide and 1-3/4 in. long. Its exact height above the base will depend upon the diameter of your coil. For the coil here described, 1 in. in diameter, the top edge of B is 5/8 in. above the base. See Fig. 78-B for shape of B, E, C before bending it, and for its dimensions. Around the end of B are tightly wound several turns of tin, making the armature or hammer, A, which should not be allowed to strike against the head of the bolt on account of residual magnetism. (See text-book.) A piece of thick paper pasted on the head for A to strike upon is best. A will probably not get near enough to the bolt to strike it, but this will depend upon how you arrange the parts.

D is a wooden piece, 1 in. high, 1 in. wide, and 3/8 or 1/2 in. thick; it is nailed to the base. Through its center is a hole for the screw-eye, S I, which is the regulating-screw. F is a piece of copper, brass, or tin, 5/8 x 1-3/4 in. It is held to the base by the screw, S, and is bent so that it presses tightly against S I. Through F is a screw, R, to hold one end of the primary coil.

149. Adjustment and Use. The battery wires should be joined to W and X, and the handles to the secondary coil at Y and Z, unless a regulator (App. 103) is used. Let us consider the primary circuit. If the current enters at W it will pass through the primary coil and out at X, after going through 5, R, F, S I, B, E, and C. The instant that the current passes, the bolt becomes magnetized; this attracts A, which pulls B away from the end of S I, thus automatically opening the circuit. B at once springs back to its former position against S I, as A is no longer attracted; the circuit is closed and the operation is rapidly repeated. B should press gently against S I, which must be screwed back and forth, until the best results are obtained. While not in use A should be about 1/8 or 3/16 in. from the bolt-head. The armature, A, should vibrate back and forth very rapidly. If this coil gives too much shock with one cell of App. 3 or 4, put a regulator (App. 103) between Y and one of the handles (App. 101).

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