The Strap, D, is made of tin. It is 4 × ½ in. before bending up the right end a little. It is fastened to the base by the screw, F, and by the other binding-post, Y. Its right end is raised enough to allow the arm, E, to pass under it, but it must press down well upon E when E is forced toward F.
The Swinging Arm or Switch, E, is also made of tin, and measures, finished, 4½ × ½ in. Its front end should be bent up a little for convenience in handling it. (See Fig. 92.) E is pivoted at G by a screw, which also binds the wire, C W. Fig. 24 shows another way to make the pivot and connection.
194. Operation. See Fig. 99 for the details of the connections of a home-made telegraph line. When you are using the line and telegraphing to your friend, the switch, E, of your instrument must be open, as in Fig. 93, and the corresponding switch on his instrument must be closed; that is, the circuit must be opened and closed at but one place at a time. As soon as you have finished, your switch must be closed. He will open his and proceed. When you have both finished, both switches must be closed. If your friend left his switch open, you could not call him over the line, as no current could pass into his sounder.
195. Batteries. As the circuit has to be left closed for hours and perhaps days at a time, so that either operator can call the other, a closed-circuit battery is necessary. (See App. 9.) A dry cell, Leclanché, or other open-circuit cell would not be at all suitable for a telegraph line, as it would soon polarize. Large Daniel cells, which are 2–fluid cells like App. 7, or gravity cells (App. 9) are the best for your line.
196. Telegraph Sounder. Fig. 94. The wood-work consists of 2 parts; the base, B, is 6 × 4 × ¾ in., and the back, A, is 6 × 5 × ½ in. A is nailed or screwed to B.
The Magnet, M, is fully described in App. 85. M is held firmly to A by cord or wire, which should pass around it near the poles and at the curved part. The wire should pass through small holes in A, and be tied at the back. Wire nails driven into A at the sides of M will keep it from moving about. The wires from the magnet coils are led to two spring binding-posts, X and Y.
197. The Armature, C, is made of a narrow piece of thin iron, about 5½ × ¼ × ⅛ in. It may be made by bending up 3 or 4 thicknesses of tin into that shape. This is the part which will be attracted by M, when the current passes, and which will make the clicks by which the message can be read. (See telegraph alphabet.) There are many ways by which C can be held near M. The figure shows how it can be done entirely with 1-in. wire nails. At the right end of C two nails are driven into A above and below C. They are just far enough apart to allow the left end of C to be raised and lowered without binding; in other words, these nails make a pivot for C to swing upon, and they help to support it at the same time. The left end of C must not quite touch the poles of M when the current passes, because the residual magnetism would keep C from dropping back into place. To adjust the armature, pass the current through M, hold C so that it will not quite touch the poles, then drive in the upper nail, 2. Put another nail, 1, below C, so that M will not have to lift C more than ⅛ or 3⁄16 in. Try the nails in different positions until C quickly rises and falls when the circuit is closed and opened. A nail, 3, driven in front of C, will keep its right end in place. No springs are needed, as gravity acts upon C instantly, bringing it to the lowest position as soon as the current ceases to flow.
198. The Battery will depend upon how much you want to use the sounder. If just to show the principle of it, almost any cell of medium strength will do, like that of App. 3, 4 or 5. A dry battery will do, but if you use the sounder much, an open-circuit battery will soon use itself up. Where much work is needed of the battery use App. 9.
The Key like App. 119 is best. Push-buttons are handy where used only for experiments, and not for the actual sending of messages.
199. Telegraph Sounder. Fig. 95. This makes a simple and efficient sounder for short lines. The base, B, is 7 × 4½ × ⅞ in. The back, A, is 7 × 4½ × ½ in.; it is nailed to B. The piece D is 4 × ¾ × ¾ in.; it is nailed to A. C is a wooden piece 1½ × ¾ × ¾ in.; it is nailed to A, and in its top is a screw, E, which is used as a regulating-screw to keep the armature, L, from touching the poles.
200. The Armature, L, is explained as App. 77. The two thicknesses of tin at F must not be too thick, or it will take too much battery power to work the sounder. If you find that it is too stiff to bend down, when the current is on, try the arrangement of App. 122, which is easier to make and regulate. The whole point depends upon the tin you have. The end of L must tap against E. A hole is punched in the part F, and a screw, G, holds it to D. L should rest about ⅛ in. above the poles and gently press against a screw or nail, V.
201. The Magnets are like App. 89. They are made as in App. 88, and held down like App. 90. These should be placed very near the back, A, so that the armature will be over them. If your yoke is not too wide the coils may rest against A. Y and Z are binding-posts like App. 46.
202. Connections. Join the coils as explained in § 125 and see § 115. Instead of a third or middle binding-post, as in Fig. 66, hold the two inside ends between a screw-head and a copper bur. The method of joining the wires for a line with two outfits, is shown in App. 124. If you have but one key, sounder, and battery, simply join the line wire to the return wire there shown. A gravity cell is best. (See App. 9.)
203. Hints About Adjusting. If you have the right spring to the part F, of the armature, you will have no trouble. It must not be so weak that it allows L to strike upon the poles, as the residual magnetism (Text-book) will hold L down after the current has ceased to pass. No springs are necessary, if your tin is right. Do not have L too far away from the poles. The distance is regulated by the position of V. If you have trouble in getting it to work see App. 122. The poles must be opposite in nature.
204. Telegraph Sounder. Fig. 96. The magnets, connections, etc., are like those of App. 121, no binding-posts, etc., being here shown. The armature is straight, however, the part F resting upon D. A hole is made in the end of F, and through this is a screw or nail, S. The hole must be large enough to allow S to pass through easily. This acts as a bearing or pivot. L is kept up against V by the rubber-band, J, one end of which passes around the end of L; to the other end of J is a thread, which is tied around a screw-eye, K. By turning the screw-eye, the band may be made to pull more or less upon L. In this way the apparatus may be regulated according to your battery. The general dimensions and explanations are given in App. 121. D is made of such a height that it will bring L about ⅛ or 3⁄16 in. above the poles.
205. Telegraph Sounder. Figs. 97 and 98. This apparatus looks a little more like a regular sounder than App. 121 and 122, but it is much harder to make and adjust. In this the lower nuts of the bolts are not sunk into the base, and the magnets are made of 2-in. bolts. If you change this and fasten them like App. 89 and 90, it will simply change the dimensions of the small parts. The sizes given are for this particular instrument.
Fig. 97 shows a perspective view, and Fig. 98 is a plan or top-view of it, with dimensions.
206. The Base, B, is 6 × 4 × ⅞ in. The magnet, M, is explained in App. 89. Its wires are attached to the binding-posts like App. 46. The armature, A, is 2½ × ¾ × ⅛ in., and made as described in App. 71. The piece, D, is 2½ × 1⅜ × ½ in., and is screwed to B from below, after the two uprights, C, are nailed to it. The uprights, C, are 2¾ × ⅞ × ½ in. They are nailed to D. The nail, N, runs through both uprights, and acts as the bearing for F to rock up and down upon. The hole for N is 2 in. above B. It must not be too loose in the holes, or F will rock sidewise, and allow A to touch one of the magnets. The upright, E, is 2¾ × ¾ × ¾ in., and is screwed or nailed to B from below. A screw, G, is put into the side of E near the top. This screw has the underside of the head filed flat, and against this the screw, L, taps when the armature is attracted. The arm, F, which carries the armature, A, is 4½ × ½ × ½ in., and is pivoted by means of N, which passes through it and the uprights C. F must swing up and down freely. The hole for N, in this model, is 1¾ in. from the armature end.
207. The armature is fastened to F by a screw, S. A copper bur is put under the head of S to aid in keeping A from rocking sidewise. Through F, and about half way between C and L, is put a screw, I, the lower end of which taps against the head of a screw, H, which is put into D. By unscrewing H a little, F will be raised, and A will be brought nearer the poles of M. The rubber-band, J, is placed over the head of I, and has tied to it a thread, O, which in turn is tied to a screw-eye, K. K screws into the end of B, and by turning it one way or the other, the tension, or pull, on J may be increased or diminished. There must be enough spring in J to pull A up after the current ceases; it must not pull so much that the magnet cannot draw A down hard enough to make a good click between L and G.
The Magnet, M, is explained in App. 89, and the construction of one bolt magnet is given in detail in App. 88. In this particular sounder the bolts are 2 in. long under the heads, thus bringing the tops of the bolt-heads about 2¼ in. above B. M is held to the base by a band of tin, T. The yoke may be screwed to B, as suggested in App. 90. This is the better plan.
208. Adjustment. You will find, although you make all of the parts with the dimensions given, that you will have to try, and change, and adjust before everything will work perfectly. A must not be allowed to touch the poles of M when it is pulled down, on account of the residual magnetism, which would keep it pulled down. Adjust this with F. The armature must not be pulled too far up from the poles of M by the tension in J; adjust this with I and H. If your battery is weak, the pull of J must be small, just enough to raise A.
The Battery. It is supposed, if you make an instrument like this, that you expect to use it for a line. In that case make a regular gravity battery like the cell of App. 9. See Fig. 99 for line connections, and Fig. 98 for plan view of this sounder.
209. Telegraph Line; Connections. Fig. 99 shows the complete connections for our telegraph line, with two complete outfits. The capital letters are used on the right side, R, and small letters are on the left side, L. The batteries, B, b, are like App. 9. The keys, K, k, are like App. 119. The sounders, S, s, are like App. 121 or 122.
210. The two stations, R and L, may be near each other, or in different houses. The return wire, R W, passes from the copper of b to the zinc of B. This is important. If the cells are not joined properly, they will not work. It is better to have the cells together, on a short line, joined in series. The line wire, L W, and the return wire, R W, may be made of insulated copper wire for short lines in the house. Ordinary annunciator wire, No. 20, is good and cheap. The kind that is double cotton wrapped, waxed, and paraffined, has about 235 ft. to the pound. You should get at least 5 ft. for 1 cent. If your line stretches from one house to another you will find it better to use iron wire. Galvanized iron or steel wire No. 14 is good. This size weighs about 100 lbs. to the mile. The return and line wires must not touch each other at any point; they must not touch any pipe or other piece of metal that will short circuit your batteries. It is best to use porcelain or glass insulators to support your wires if the line is long; but for short lines, where you use a return wire, you may support the wires upon poles or trees by means of loops made of strong cord or wire.
211. Operation. Suppose R (right) and L (left) have a line. By studying Fig. 99 you will see that R's switch, E, is open while e is closed. The whole system, then, has but one place where the circuit is open. As soon as R presses his key, K, the circuit is closed, the current from both cells rushes around through K, S, L W, s, k, b, R W, and B. This magnetizes the bolts of both S and s, and their armatures come down with a click upon the regulating-screws, where they remain as long as the current passes. As soon as R raises his key the armatures rise, making the up-click. R can, in this way, regulate the time between the two clicks. If he presses K down and lets it up quickly, the two clicks that his friend L hears from s are close together; this makes what is called a dot. If R holds K down longer, it makes a longer time between the clicks for L to hear, and this makes a dash. R, of course, hears his own sounder, which is making the dots and dashes also.
As soon as R has finished, he closes his switch, E. L then opens his switch and proceeds to answer. Both E and e should be left closed when you are through talking.
(Read § 194, 195, and study what is said in App. 9 about the gravity cell to be used on such a line.)
212. Telegraph Alphabet. The letters are represented by combinations of dots, dashes and spaces. A dot is made by pressing the key down, and raising it at once; that is, the key is raised as soon as it strikes. This makes the letter E. The dash is made by pressing down the key, and allowing the current to pass about as long as it takes to make 3 dots; this makes the letter T. A long dash for L should take about as long as for 5 dots. Spaces occur in a letter and between words. To make a dash you hesitate while the lever of the key is down, to make a space, you hesitate while the key is up. H is made with 4 dots without hesitation or space. By putting a space between the dots the letter &, Y or Z is made according to the position of the space. Notice that letters containing dashes do not contain spaces. A space is really the opposite of a dash. The letters C, E, H, I, O, P, R, S, Y, Z, and & are made entirely of dots or of dots and spaces.
You should notice that several letters are the reverse of others; A is the reverse of N, B of V, D of U, C of R, Q of X, and Z of &. The student should study some book upon telegraphy, if he desires to become expert. Punctuation marks are left out of the alphabet here given, as boys will find very little use for them.
CHAPTER XV.
ELECTRIC BELLS AND BUZZERS.
213. Electric Buzzer. Fig. 100. A buzzer is, in construction, very similar to an electric bell; in fact, you will have a buzzer by removing the bell from any ordinary electric bell. They are used in places where the loud sound of a bell would be objectionable. As the buzzer is easier to make than a bell, we shall discuss it first.
214. The arrangement of the parts, (Fig. 100), is very much like that of the sounder of App. 121, Fig. 95. The armature is, in this case, a vibrating one and acts on the same principle as the automatic interrupter on App. 100, which you should study. (See § 148.) The general dimensions may be taken from App. 121. The base, B, in this case is about 1 in. wide. D also is made 1 in. wide. H is 1 × 1 × ½ in., and is nailed to A. Through its center is a hole for the regulating screw-eye, I. The end of I presses against F. The exact position of H will have to be determined after the magnets are in place. The armature, L, should be about ⅛ or 3⁄16 in. above the poles. They are not allowed to strike the poles, as a screw, E, regulates that. (See § 203). Y and Z are two binding-posts, like App. 46. To these are connected the battery wires. The strip of tin or copper, which forms Y, is cut like a letter T there being three holes in it, one near the end of each arm. The screw-eye, 2, and the screw, 3, are put through the horizontal part of the T, and the regulating-screw, I, passes through the hole in the vertical part which springs up against I, thus forming an electrical connection between Y and I. The magnets are made and fastened as in App. 89.
215. Connections. The inside ends of the magnet coils, (§ 123), are fastened between a screw-head and a copper bur, S. One outside end goes to Z, and the other under the screw, G, which holds F to D.
216. Adjustment. The part, F, and the screw, E, must be just high enough to keep L from striking the poles of M. If F is too weak, it will bend down to M. If F is too strong, it will take too much battery power to run it. In case there is not strength enough in F to quickly raise L when the current ceases to pass, arrange a screw-eye and rubber band as shown in Fig. 96. I should be slowly turned one way or the other, until it touches F just right to allow L to vibrate back and forth rapidly.
217. Operation. We shall suppose that you have all parts adjusted and the battery wires joined to Y and Z. If the current enters at Z, it will fly around through the coils, through G, F, up I, through the T-shaped tin and out at Y. The current was in L, but it could not get out at any other place than at Y. As soon as the bolts were magnetized, L was forcibly drawn down, pulling F away from I, thus opening the circuit. As the bolts were no longer magnets, F sprang right back to I, the current passed long enough to re-magnetize the bolts. This operation was rapidly repeated.
218. Use. If you wish to use the buzzer simply to call some one occasionally, a dry battery or Leclanché cell is best. This apparatus is good to work a gravity cell when it needs regulating.
219. Electric Bell. Fig. 101. Before making this bell, carefully read the directions and explanations given for the electric buzzer, App. 125. The parts are very much alike in the two instruments, and most of the lettering of them has been made the same in the illustrations. If you look at Fig. 101 from the side, with the letters M and Q at the bottom, you will see that this bell is merely a modified form of App. 125.
The Base is 7 × 5 × ½ in. To the upper end of this is nailed the cross piece, D. To D are fastened the binding-posts.
The Parts, F, G, H, I, J, K, L, M, N, P, Q, are the same as explained in App. 121 and 125.
The Magnet is fastened to the base by a tin strip, C, which is screwed down at both ends. By nailing a strip, like D, along the left side of the base, the magnet may be fastened to this. This strip would take the place of the base of App. 125.
The piece, F, of two thicknesses of tin, is made longer than it was in App. 125; in fact, it projects through L and forms the part N. To the lower end of N is fastened a large bullet. Hold the cutting-edge of a strong knife-blade upon the bullet, and with a few taps of a hammer drive the blade into it to make a gash.
Put the end of N into the cut, then hammer the bullet so that N will be pinched. If you have no bullet, cut a long strip of tin, about ⅜ in. wide, and wind this about the end of N to serve as a ball.
The Bell, E, may be taken from an old alarm-clock. This is not screwed directly to the base, as it would not ring well. After you have the ball, O, properly fixed, hold E, so that O will strike it near its rim; then cut a piece of wood about ⅝ × ⅝, and long enough to put under E, to raise its rim to the right place. This piece must be screwed to the base from the underside, and on to its top is placed the screw which passes through the bell. In other words, E is mounted upon a rod which is fastened to the base.
The Adjustments are made as in App. 125. By bending N a little, O can be made to tap E properly.
The Battery for a bell that is to be used much should be an open circuit one, such as the Leclanché, or the ordinary dry batteries. It is cheaper to buy a dry battery than it is to make one suitable for bells. A and B show wires that lead to the bell from the battery. One of the wires should be passed through a push-button.
220. Electric Bell. By arranging the buzzer of App. 125 with a bell, you can use the same for an electric bell. The part, F, should be made long enough to extend entirely through L, and project beyond L for about 2 in. To the end of this is fastened a large bullet, or a band of tin. (See App. 126.)
221. Combination Buzzer and Telegraph Sounder. Fig. 102. This apparatus is good for experimental purposes, where you do not wish to go to the trouble to make two separate pieces. For the dimensions and explanations see App. 121 and 125. There is but a slight change in App. 125 to make this.
222. Connections. The inside ends (§ 123) of the magnet wires are fastened together at S. The outside ends are joined to the two binding-posts, Y and Z, made like App. 46. A wire, P, joins Y with the screw in T, which is a piece of stiff tin or copper, which presses down upon the top of I. In this way a connection may always be had between I and T. A wire, R, joins F electrically with X; it is held under the head of the screw, G. (See App. 125 about adjustments.)
223. Operation. When you wish to use the apparatus as a buzzer, join your battery wires to X and Z. If the current enters Z, it will pass through the magnet coils out to Y, through P, T, I, F, and R to X. If you use it as a telegraph sounder, join the battery wires to Y and Z. The current will then pass simply through the coils; it will not bother to go into P, F, etc., as it has no place it can escape. If used simply for experimental purposes almost any cell of sufficient strength will do. If for telegraph, use App. 9; if for buzzer, use an open circuit cell, as, for example, a dry cell.
CHAPTER XVI.
COMMUTATORS AND CURRENT REVERSERS.
224. Commutators and Current Reversers are useful in some experiments, as, for example, those with tangent galvanometers (App. 116, 117), in which readings are made with the current passing around the coil in one direction, and again made at once with the current reversed. The use of commutators on motors and dynamos should be understood. The reversers herein shown are, of course, not at all like those used on motors. Current reversers are used in connection with the needle-telegraph and many other instruments.
225. Current Reverser. Fig. 103. The base is 5 × 4 × ⅞ in. To this are fastened four metal straps, A, B, C, and D. These may be made of brass, aluminum, or even of tin. If made of tin, use one thickness of metal for C and D, and two thicknesses for A and B. Each strap has two ⅛ in. holes punched in it, their positions being shown by the screw-heads and screw-eye binding-posts.
Construction. C is 3¾ × ½ in. Fasten this to the base first. At the left end is a small screw, while the right end is held down by the binding-post, W. The keys, A and B, should have quite a little spring to them. These are cut 5 × ¾ in. The front end of each is bent over a little (see the key App. 118, Fig. 92) so that they may be more easily grasped. The length after bending will be less than 5 in. The front ends should be raised from the base (Fig. 92) so that they will not touch C, unless pressed down. The ⅛ in. holes in the end of A are about ¾ in. apart, one being used for a screw to hold it to the base, and the other for the binding-post, Y. The strap, D, is 3¾ × ½ in. It is fastened at one end by a screw, and at the other end by X. D is bent about ¾ in. from each end, so that its middle part stands above the base about ¼ in. The straps, A and B, press up against D, unless they are held down with the hand.
226. Connections. W and X are joined to the poles of the battery to be used. Y and Z are joined to the apparatus in which the current must be passed in one direction, and then in the opposite direction. A tangent galvanometer, or a needle-telegraph instrument, for example, may be connected with Y and Z.
227. Operation. Suppose that the battery current enters at W. As long as both keys are raised, the current can go no farther. Now, imagine that we press A down solidly upon C, the current will pass along A, which does not now touch D, out through Y into the galvanometer, back to Z, into D, and to the battery again; that is, the current will enter the galvanometer from Y. Now, suppose that we let A spring up against D again, and press B down, the current still coming into W from the battery; the current will pass along B, out through Z, into the galvanometer, back to Y, through D, and back to the battery. It is evident, then, that the current can be made to pass out of Y or Z to the galvanometer at will by pressing down A or B.
228. Current Reverser. Fig. 104. The wooden base is 7 × 5 × ⅞ in. To this are fastened two brass or tin straps, C and D, 5 × ½ in. They are fastened at the front ends by screws, S, while the binding-posts, Y and Z, hold the other ends solid. X and W are two screw-eye binding-posts (App. 45). The small square piece of wood, T, is 3 × 3 × ½ in. Through the corners of T, and in positions so that they will be directly over C and D, are put four screw binding-posts, 1, 2, 3, 4 (App. 41). The screws, however, pass entirely through T, and stick out about ¼ in. on the underside of it. The wire, A, connects W, 1 and 4, while the wire, B, connects X, 2 and 3. A and B must not touch each other where they cross on the top of T. N is a wire nail that serves as a handle. If we were to place T, holding the four corner screws, upon the straps, C and D, it is evident that all the screws would touch the straps, if they were properly adjusted. We must fix things so that two only can touch the straps at a time. Put a screw, Q, through the center of T, from the bottom, so that it will stick out of the bottom more than the screws, 1, 2, etc. The screws, 2 and 4, will be lifted from C and D when the handle, N, is pressed down. By raising N, the top, T, can be made to rock up and down upon Q as a pivot. By lifting N far enough, 2 and 4 will be pressed against C and D, while 1 and 3 will be raised. A spring, R, is shown joined to T and to the base. This will hold the screws, 2 and 4, down upon C and D, unless N is pressed down.
229. Operation. We shall first suppose that the spring, R, is holding 2 and 4 in contact with C and D; 1 and 3 will, of course, be held up in the air. Imagine that we have a galvanometer connected with Y and Z. If the battery current enters at W, it will pass along A to 4, before it can find a chance to escape. It will pass through 4 into D, and into the galvanometer by way of Z, then back by way of Y, up 2, and out to the battery from X. If we now press the handle, N, down, the current will pass from W to 1, down 1 through C and Y to the galvanometer. It will return to the battery by way of Z, D, 3, B, and X. The current can then be rapidly reversed by raising and lowering N.
CHAPTER XVII.
RESISTANCE COILS.
230. Resistance Coils. Fig. 105. For experiments in resistance (See text-book), a set of standard resistances is necessary. There are many ways in which the resistances may be made; you can arrange them upon a long board, upon a rack, or wind the wires around spools. We generally speak of resistance coils. The Ohm is taken as the standard. If you use copper wire, you may take 9 ft. 9 in. of No. 30 insulated wire as your standard Ohm. You could, of course, take any other length of any size as your standard, but it will be best to make your coils with a certain number of Ohms resistance. If you have no No. 30 wire, you may use 39 ft. 1 in. of No. 24 insulated copper wire for 1 Ohm. (See wire tables in text-book.)
231. To avoid the magnetic effect (See resistance coils, in text-book), the wire should be measured off, then doubled, before winding it upon the spools. The wire may be held to the spool with paraffine. Fig. 105 shows how the doubled wire looks on the spool, a few turns only being shown. Do not use any nails or other iron in connection with the coils proper.
232. By making 4 coils having, respectively, 1, 2, 2, and 5 Ohms resistance, you will be able to use any number of Ohms from 1 to 10. These will be very handy in connection with a "Wheatstone's bridge" for comparing resistances. (See text-book for experiments). The coils should be mounted upon a base with proper binding-posts, so that one or more coils can be used at a time. (See App. 132.) For the 2–Ohm coil use, of course, twice as much of the same kind of wire as for the 1–Ohm coil.
233. Resistance Coils. Fig. 106. The construction of one coil is given in App. 131. To have the set of coils so that they can be easily used, place the spools upon a base which, in the model, is 8½ × 4 × ⅞ in. The spools are 1¾ in. apart, center to center, and should be glued to the base. Fig. 106 is a plan of the apparatus. U, V, etc., are binding-posts like App. 46. The figures between them show how many Ohms resistance there are in the coil above. The coils A, B, C, D, and E are wound respectively for 1, 2, 2, 5 and 10 Ohms.
234. Connections. If you join a Wheatstone's bridge, for example, with U and V (Fig. 106), the resistance added will be but 1 Ohm; if you join with U and W, the coils A and B will be in the circuit and make 3 Ohms resistance; if V and X, 4 Ohms; if V and Y, 9 Ohms; if U and Z, the whole, or 20 Ohms.
235. Resistance Coils. For use in some experiments in comparing the resistance, diameter, lengths, etc., of wires (See text-book), it is very handy to have coils made a certain number of meters long. (The meter is a French unit of measure and represents 39·3705 of our inches). German-silver wire has a much greater resistance than copper wire of the same size and length.
(a) Make a coil (See App. 131 for method) containing 1 meter of No. 30 German-silver wire.
(b) Make a coil with 2 meters No. 30 German-silver wire.
(c) Make one with 2 meters of No. 28 German-silver wire.
(d) Make one with 20 meters of No. 30 copper wire.
The above wire must be insulated if it is to be wound upon spools. Bare wire may be arranged on boards or racks so that the current may not be short circuited.
CHAPTER XVIII.
APPARATUS FOR STATIC ELECTRICITY.
236. Static or Frictional Electricity. There are many interesting and instructive experiments in this branch of electricity. All that can be done here is to explain a few pieces of simple apparatus to show the presence of static electricity, it being taken for granted that you know how to produce it, and that you have some book of simple experiments.
237. Electroscopes are instruments for showing the presence of static electricity.
238. Thread Electroscope. A piece of ordinary thread may be used for this purpose. Tie one end of it to the back of a chair or other support.