239. Pith-Ball Electroscope. Fig. 107. The pith from elder, corn-stalk, milk-weed, etc., is very light and porous. When this is tied to the end of a silk thread, we get the pith-ball electroscope, so much talked about in nearly every text-book on physics. The upper end of the thread may be tied to any suitable support. Fig. 117 shows a book, lead pencil, and a small weight to hold the pencil steady. The thread is tied to one end of the pencil.
240. Support for Electroscopes, etc. Fig. 108. Glue or nail a spool, S, to a wooden base, B, measuring about 4 × 5 in. Wrap some paper around a 7 in. length of ¼ in. dowel, D, to make it fit the hole in S. Wind one end of a wire, W, around the top end of D. To the outer end of W tie a silk thread, S T, on the lower end of which may be tied a piece of pith or material to serve as an electroscope.
241. Carbon Electroscope. Carbon will be found to make a most excellent electroscope, as it is light and a good conductor of electricity. Light an ordinary match and let it burn until it is charred through and through. The black substance remaining is carbon. Tie a small piece of the carbon, about ¼ in. long, to one end of a silk thread, and support the thread as in Fig. 107 or 108.
242. Pivoted Electroscope. Fig. 109 and 110. Fold a piece of stiff paper double, then cut it into the shape shown. It should be about 3 in. long and 1 in. wide when opened out. A hole, B, about ½ in. in diameter should be cut in it while folded. A piece of paper, C, should be pasted to A, so that its top, where it is creased, will be about ⅛ in. above the top of A. The support consists of a pin, E, stuck through a cork, D. Balance the paper on the pin, which passes up through the hole, B. An electrified body brought near this apparatus will make it whirl around very decidedly.
243. Fancy Electroscope. Fig. 111. Fold a piece of stiff paper double, then cut out some fancy-shaped figure, as suggested, and draw the face, clothes, etc., to suit. This being folded through the center for cutting, it can be balanced upon a pin-point as explained in App. 138.
244. Box-Cover Electroscope. Fig. 112. A pasteboard box-cover, balanced upon a pin, makes a fairly good electroscope, although it is not nearly so sensitive as App. 138. The pin may be stuck in the upper end of the dowel, D, shown in Fig. 108.
245. Leaf Electroscope. Fig. 113. This is a very sensitive instrument, and can be used to tell the kind of static electricity on a body, as well as the mere presence of it. (See experiments in text-book.) The lamp chimney acts as a support for the leaves, L, and it protects them from currents of air. A tin box-cover, C, has a small hole punched through its center. Through this is pushed one end of a wire, W. This may be a hairpin, straightened. The upper end is bent over at right angles, after passing it through the hole. The lower end is bent as shown. On this horizontal part is fastened the leaf. These should be made of aluminum leaf, or of Dutch metal. The former will stand more rough handling than the latter. Goldleaf is used for very sensitive instruments. It is a little too delicate for unskilled hands.
246. To cut the aluminum leaf, place it between two pieces of paper, then cut paper and all into the desired shape. The piece should be about 3 in. long and 1 in. wide. Fold this across the middle, and stick it to the underside of the wire (Fig. 113). Saliva will make it adhere to the wire, if you have nothing better.
247. To Show Where a Charge of Static Electricity Resides. Fig. 114. This shows a tin baking-powder box placed upon a hot tumbler. A moist cotton thread is hung over the edge of the box. (See experiments in text-book.) The box will become charged by touching it with a charged body. The thread will show whether the charge resides upon the inside or upon the outside of the box.
248. Support for Electrified Combs. Fig. 115. In the study of static electricity, ordinary ebonite combs can be used to great advantage. A bent hairpin will serve as a cradle to support them. A silk thread may be tied to the wire, but a narrow silk ribbon is better than thread, as it will hold the comb steady.
249. An Electric Motor is really a machine. If it be supplied with a proper current of electricity, its armature will revolve; and, if a pulley or wheel be fastened to the revolving shaft, a belt can be attached, and the motor made to do work. There are many kinds of motors, and many simple experiments which aid in understanding them. All that can be done here, however, is to show how to make simple motors. (See text-book for experiments.)
250. Electric Motor. Fig. 116, 117. Fig. 116 shows a plan or top view, and Fig. 117 shows a side view, with a part of the apparatus removed, for clearness.
The base, B, is 5 × 4 × ⅞ in. The upright, U, is 3½ × 1½ × ½ in., and is nailed or screwed to B. The binding-posts, X and Y are like App. 46. 4 is a screw binding-post.
251. The Field-Magnets, as the large electro-magnets on a motor are called, are made of 5⁄16 machine-bolts, 2½ in. long. The washers are 1½ in. apart inside. (See App. 88 for full directions.) The bolt cores are 2 in. apart, center to center. (See App. 89.) The tin yoke, D, is made like App. 71, and it is fastened to the base, like App. 90. The hole for the screw, however, is made a little to one side of the center, so that a dent can be made at the center for the bottom of the shaft, 8, to turn in. Make the dent with a center punch. The yoke is fastened to B, so that one edge of it is 1½ in. from the back edge of B. (Fig. 116).
252. The Armature, A, is made of 6 or 8 thicknesses of tin, 2½ in. long and ¾ wide. (See App. 71.) In its center is punched or drilled a ¼ in. hole, so that it can be slipped onto the ¼ in. "sink-bolt," 8. If you have taps you can make the hole a little smaller than ¼ in., and thread it so that it will screw onto 8. A must be heavy enough to revolve a few times when once started. It is pinched between two nuts, 9 and 11, so that it just clears the poles when it turns. (See App. 145 for another form of armature.)
253. The shaft or axle, 8, is made of a "sink-bolt" that is 3 in. long and ¼ in. in diameter. These sink-bolts are threaded over their entire length, and are furnished with two nuts, 9 and 11, Fig. 117. File or grind the end of 8 to a point, so that it will turn easily in a dent made for it in the yoke, D, or in a dent made in another piece of tin fastened over the yoke. The shaft is held in a vertical position by the arm, C.
254. The Arm, C, is made of 2 or 3 thicknesses of tin. It is 3 × ¾ in.; it has in one end a hole for the shaft to revolve in easily, and in its other end a slot is cut. A screw-eye and bur are used to hold C to the upright, U. By this means the shaft can be moved and regulated as to position.
255. The Commutator, 9, (Fig. 117), is made of one of the nuts furnished with the shaft. Two of its corners are filed or ground off, so that it has the shape shown at the right, in Fig. 117. The copper wire, 10, rubs against 9, as the pointed part of it comes around. 10 is really a "brush," and carries the current into 9 at the right time.
256. Connections. Join the two inside ends (§ 123) of the coils to 4. The outside end of 2 is joined to X; the outside end, 7, of the other coil, 6, is carried up under or around the screw-eye, S I, and then its bare end reaches out and gently scrapes against the top of the shaft, 8. The wire, 10, leads from Y to the back of the base, where it is carried up to a screw, 12, which holds it to U. Its bare end reaches out to gently scrape against the commutator, 9, when it swings around. This wire, 10, should not press against 9 during the entire revolution.
257. Adjustment. Suppose the current enters at X. When the "brush," 10, presses against the commutator, 9, the current passes through X, 1, 2, 3, 4, 5, 6, 7, down 8 to 9, and out through 10 to Y. (The current, of course, goes down into D and into the bolt-cores also; but it can go no farther, if the coils are properly insulated, and A is not allowed to touch the cores. It is better to have the end of the shaft rest upon a piece of glass, having a slight depression made with a file, or in a dent made in tin which rests upon wood, the tin having no connection with D.) If 10 should continue to press against 9, the current would continue to pass, and A would be held firmly in place, directly over 2 and 6, and, of course, the shaft could not revolve. If, however, the brush leaves 9 (See plan of 9 at side of Fig. 117), just as A gets over the coils, or an instant before it gets there, the weight of A will carry it beyond the coils. No current should pass again, until A is at least at right angles to a line drawn through the center of the coils. If the current again passes, the ends of A will be attracted by the bolt-cores.
In other words, the current should pass a little less than one-half of the time, and this is divided into two parts. Suppose you start A with your finger; the current should be shut off automatically just before the center of A gets over the center of the bolt-cores. A makes ¼ of a revolution without current, and just after it gets beyond this, the current passes for nearly ¼ of a revolution, which brings the ends over the poles again. The next ¼ of a turn it has no current, because the flat side of 9 is opposite the brush, 10, as during the first ¼. The last ¼ the current passes again. The exact position of the commutator will depend upon the way you arrange the brush. The positions of 9 and 10 can be found by trial, so that the circuit will be promptly opened and closed at the proper moment. Start the motor by turning the armature.
258. Batteries. The amount of power needed will depend upon how well you make the motor. One cell of App. 3 or 4 will run a well made one, but it is better to use 2 cells. Join the wires to X and Y.
259. Armature for Motors. Fig. 118 shows another form of armature that may be used for small motors like App. 144; in fact, you may find that this form is easier to make than that of App. 144. M is a 5⁄16 machine screw, 1½ in. long, 9 being the nut furnished with it. 9 is filed as explained in § 255, and forms the commutator. C is the arm (§ 254). A is the armature (§ 252). A is held firmly in place between the spool, E, and 9. S is a set-screw which passes through E, and holds the piece of ¼ in. dowel, F, in place. N is a needle-point fastened in the end of F. N revolves in a dent made in a piece of tin, H, which rests upon a wooden strip, G. G is cut away on its underside, so that it will straddle the yoke, D, Fig. 117; it is nailed to the base. This is given as a suggestion. By making F a little longer, N can turn in a dent made in the yoke, below G.
260. Adjustments. M, being 5⁄16 in. in diameter, will screw solidly into the hole in E. Place 9 upon it first, then A, and screw it about ½ way into E. 9 will serve as a lock-nut by turning it so that it will pinch A and hold it firmly against the top of E. F should reach half way into E. Put N in place after you have H and G arranged. You can then cut the upper end of F at such a place that it will bring A about ⅛ in. from the top of the magnet-cores. Paper wrapped around F will make a good fit in E. The current should enter M and leave 9, as fully explained in App. 144. (See § 257).
261. Electric Motor. Fig. 119, 120, 121, 122. Fig. 119 shows a front view, and Fig. 120 a side view of the whole motor. Fig. 121 shows the part that revolves, and includes the shaft, armature and commutator. Fig. 122 shows a section of the commutator. All the dimensions are taken from a model. You can modify the size to suit.
262. Wood-work. The base is 7 × 5 × ⅞ in. The uprights, U, are 3½ × 1 × ¾ in. They are screwed or nailed to the base from below, their 1-in. sides being towards you in Fig. 119. They are 4¼ in. apart, inside, in this model. The piece, A, is 2½ × ⅞ × ⅝ in., and is cut away on the underside to straddle the yoke. Fig. 118 is a suggestion as to its shape. A is screwed or nailed to B.
263. Tin-work. The horizontal arm, T, is made of 3 thicknesses, and holds the shaft in a vertical position. T is 6¼ × ¾. In its ends are slots, and in its center is a hole so that the ¼ in. shaft can revolve easily, but not too loosely. The slots allow an adjustment, the screws, S, holding T to U. The shaft rests in a dent made in a piece of tin which is tacked to A. The yokes are elsewhere described.
264. Field-Magnets. In this model they were made of 5⁄16 bolts, 2 in. long, placed 2 in. apart center to center. The washers are 1⅛ in. apart inside. (See App. 88 for full directions.) App. 89 and 71 should be studied. Except in size, they are made as in App. 144. They have 8 layers of No. 24 or 25 wire.
265. The Armature, Fig. 121, on this style of motor consists of a regular horseshoe electro-magnet, made in the same general way as the field-magnets. The electro-magnets, 12 and 16, are smaller, however, than the field-magnets. The cores are ¼ in. stove-bolts, 1¼ in. long under the head. They are placed 2 in. apart, center to center. They are insulated and wound as fully explained in App. 88. These ¼ in. bolts require a change in your winder. (See App. 147 for this.) If you wish to use 5⁄16 bolts, you may use the same axle for your winder as before. The washers are ⅝ in. apart, inside. The cores are wound with 4 or 6 layers of No. 24 or 25 wire. This makes them about ¾ in. in diameter. They are held in a tin yoke, 14, made of 5 or 6 thicknesses of tin. 14 is 3 × ¾ in., and has 3 holes punched in it. The two outside holes are 2 in. apart. Through these pass the bolts, which are held firmly by the 2 nuts. The shaft, S B, is a sink-bolt, 3 in. long, and ¼ in. in diameter. (See § 253.) The inside ends (§ 123) of the coils should be firmly twisted together or held under the top nuts to make a good connection between them.
266. The Commutator is in two parts, which must be insulated from each other. The 2 sections are made out of thin tin or copper in the shape of an inverted T, as shown at 10, Fig. 121. The arms of the T are about ⅜ in. wide, the horizontal ones reaching about half around the spool, E. The vertical arm reaches over the top of E, and is held down by a small screw, J. The sections, 10, must not touch the shaft. The outside wires (§ 123) of 12 and 16 are fastened under these screws, J, and they must not touch the shaft. Bend the tin sections so that they will be as nearly round as possible. The spool, E, has been sawed off so that it will go between the field-magnets. Wind paper around the shaft to make it fit solidly into E. S is a small screw that holds E in place, if the paper does not hold it tight enough.
Fig. 122 shows a section of the spool and tin sections with the brushes pressing against them. The sections do not touch each other, and the brushes touch opposite sections. It is evident, then, that the current must pass through the coils 12 and 16 in order to get from one section of the commutator to the other, provided you have no short circuits through the shaft or elsewhere. The slots in the commutator must be directly under the center line of the yoke, 14, as seen in Fig. 121.
267. The brushes, 9 and 19, Fig. 120, are made of very thin tin or copper. They are cut to the shape shown, the narrow part being about ⅛ in. wide, and long enough to reach at least to the center-line of the apparatus. The foot, or bottom part of the brushes, should be about 1¼ × ¾ in. These are used to fasten them to the base and to make connections. If you have no thin metal for brushes, use copper wires, and arrange them so that they will press gently against the commutator.
268. Connections. The inside ends (§ 123) of the field-magnets are held at 4. The outside end of coil 2 is joined to X, and that of coil 6 to 8, the foot of the brush which presses against 10. The section, 10, of the commutator is joined to 11, the outside end of coil 12, its inside end being fastened to the inside end of coil, 16, either by twisting them together, or by fastening them under the top nuts of the armature yoke, 14. The outside end of coil 16 is joined to the other commutator section, 18. The brush, 19, completes the circuit. In the foot of 19 is the binding-post, Y.
If the current enters at X, it will pass through 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and out at Y, provided 10 and 18 are in contact with 9 and 19. Be careful not to have any short circuits. If, for example, the wire 7 touches 4, or if 3 touches 8, or if the wires 11 and 17 touch the shaft, your current will not pass where you expect, and you will have trouble.
269. Adjustment. The armature cores should just clear the poles of the field-magnets as they turn. This must be regulated by the thickness of A and the position of the nuts on the shaft, S B. The slots in the commutator must be under the center of the yoke, 14. The brushes, 9 and 19, must touch 10 and 18, but not so hard that they will stop the motor. Wire brushes are more easily adjusted than tin or sheet-copper ones. The tin arm, T, must hold the shaft properly. The point of the shaft must allow it to turn easily. The motor will turn clockwise if the attachments are made as shown. Use 1 or 2 good bichromate cells, like App. 3 or 4.
270. Operation. The current will pass through the field-coils in the same direction, as long as the battery wires are not changed. The current is reversed in the armature-coils every time the brushes change from one section to the other of the commutator; that is, it flows in one direction during one-half of a revolution, and in the opposite direction during the other half. This reverses the poles of the armature-magnets every ½ revolution. (See text-book for full explanations and for simple experiments with electric motors.)
271. Attachment for Winder. In winding small electro-magnets for armature, etc., in which cores are used that are not 5⁄16 in. in diameter, your winder will have to be slightly changed. Its 5⁄16 stove-bolt will have to be removed, and a ¼ in. one put in instead. This may be done by making a handle for the ¼ in. bolt. To keep this from wobbling in the 5⁄16 hole, wind stiff paper around the bolt until it fits quite tightly. The whole winder is explained as App. 93.
272. Graduated Circles. Fig. 123. For compasses (App. 67), and for use in connection with tangent galvanometers (App. 116), a graduated circle is necessary. Fig. 123 is a reduced drawing from an original that is 4 in. in diameter. The long lines are 10 degrees apart, the smallest divisions shown being 5 degrees apart. Single degrees can be determined with considerable accuracy with the eye.
To divide the circle. Divide the circumference into 4 equal parts; these will be 90 degrees from each other, there being 360 degrees in every circle. Divide each quarter into nine equal parts with a pair of dividers; these will be for the long lines, 10 degrees apart. Divide each of these into two equal parts. If you are used to drawing, you can divide the circle still more, but 5–degree divisions will do.
273. Adjustable Table. Fig. 124. A table that can be raised or lowered is useful. The one shown at D, Fig. 124, is used for the galvanometer of App. 117. The dimensions are given in the figure. The upright piece, U, is fastened to D with brass screws, not with nails, as these would affect the needle. It is placed at one side of D so that the compass needle placed in the center of D will also be in the center of the wire coils when used in App. 117. The table is fastened in any position by a screw-eye, S I, which presses a copper washer, W, against U. S I works through a narrow slot, S, and screws into the back of the galvanometer. By making S longer, the table may be used for other laboratory purposes, if it is joined with some other form of standard.
274. Glue Pot. If you have occasion to use glue, you can make a good glue pot out of 2 tin cans, one being placed inside the other. Put ¼ teacupful of glue in the inside can. If you have time, cover it with cold water, and let it soften. If you are in a hurry, cover it with hot water. Set this inside can into the other, in which you have boiling water. Do not let the water boil over. The solder will not melt from ordinary tomato cans, if you keep water in them. Thin the glue with a little hot water until it drips from the brush in drops. Have the glue hot and fairly thin, and apply quickly. Hold the pieces of wood together by pressure until the glue hardens.
275. Paraffine Paper and Cardboard are extremely useful for insulating purposes. The paraffine used in candles will do, if you cannot get it in block form. While ordinary paper will do for simple apparatus to wind about coils, etc., you will find that paraffine paper can be handled very rapidly. To melt the paraffine you should use a double boiler, or one made of a shallow basin set in a pan of water. The water should be boiled. This will melt the paraffine in the basin. Strips of paper just passed through the melted paraffine will become soaked, and the paraffine will quickly harden in the air. Allow thick cardboard to soak for a minute or two, to drive out all the air. This makes excellent washers for electro-magnets. (See § 119.) To make one piece of this paper stick to another, merely pass a clean hot nail over the two where they lap. To hold coils of wire together, or to wooden bases, use a few drops of paraffine applied with a large hot nail.
276. Caution. Do not heat paraffine directly upon the fire or over a burner, unless you watch it constantly. It will burn if its temperature is raised too much. It is better to heat it with steam, as you do glue.
277. Battery Jars. For small cells, use glass tumblers. Ordinary glass fruit jars are good. Even earthen bowls may be used, and for large cells—if you have nothing better—you can use small earthen crocks or jars.
278. Glass Bottles can be cut off so that they will make excellent jars. If you have thin bottles, you can cut them with strong cord. Tie one end of the cord, which should be 5 or 6 feet long, to a door knob or to a solid post. Tie the other end around your body. Make one complete turn of the cord around the bottle where you wish to cut it; draw the cord tight by stepping back, and with both hands draw the bottle back and forth vigorously many times, so that the cord will rub it hard and make it very hot. Do not let the cord move lengthwise upon the bottle. This will make a circle around the bottle that is very hot. Immediately plunge the bottle into cold water, the colder the better. Use ice-water, if you have it. If you produce heat enough, the bottle should crack all the way around very neatly. File off any sharp corners and edges with a wet file.
279. A hot iron can be used with success to cut off a bottle. File a deep groove first, hold the red-hot iron first on one side of file mark and then on the other to start the crack. You can lead the crack wherever you wish by keeping the iron about ⅛ in. ahead of it.
280. A small gas-flame will be much better than a hot iron, and you may easily use it, if you have glass tubing, rubber tubing, etc., in your shop. Draw out the glass so that the gas will burn in a fine needle-like flame about 1 in. long. Keep the point of the flame about ¼ in. ahead of the crack. The glass tube should be held in a rubber tube connected with the gas pipe.
281. Your Workshop. If possible, keep all your work, tools and apparatus in one room, and lock the door when you leave.
The work-bench may be made of an old kitchen table, or of a strong, large box. The tool chest may be made of any clean box about the size of a soap box. Shelves can be made by setting soap or starch boxes on their sides, one above the other.
282. The tools needed are generally mentioned in the proper places, under the directions for construction. It is better to buy your tools as required, than to buy too many at once, some of which you may not need. If you have absolutely no tools, not even a saw or hammer, you will be obliged to buy or borrow, although a great deal can be done with a good knife. Do not be satisfied with rough-looking pieces of apparatus.
There are a few important tools needed for this work. While substitutes can be found for most of them, the boy who has access to a wood-working bench and tools will be able to do better and more rapid work than the boy who has no such tools.
283. List of tools. The following tools are needed, if rapid, accurate work is desired:
(1.) Lead pencil. (2.) A rule, divided into sixteenths for measuring. A straight foot rule will do,—cost one cent. (3.) Steel point for scratching lines on tin and copper. A stout needle-point is just the thing. (4.) An awl for making holes in wood; one that is a little less than ⅛ in. in diameter is best. (See App. 25.) (5.) A try-square with a 6 in. blade, so that you can mark out your apparatus with square corners. You can use a square-cornered box or piece of pasteboard, if you have no try-square. (6.) Chisels are very useful, but you can do wonders with a good sharp knife. (7.) Screw-driver. Do not use a good knife-blade for a screw-driver. (8.) A saw, one with teeth that are not too coarse is to be preferred. (9.) A plane is extremely useful to make your wood-work smooth and neat; but a great deal can be done with the sharp edges of broken glass, followed by a good rubbing with fine sand-paper. (10.) A brace and a set of bits may be needed in 2 or 3 cases, but nearly all of the holes can be made as in App. 25. (11.) Punches for sheet-tin, etc., will save much time. (See App. 26, 27.) For small holes in binding-posts, etc., use a flat-ended punch, ⅛ in. in diameter. You should have one ¼ or 5⁄16 in. in diameter, if you make your yokes, armatures, etc., as in Chapter VIII. A blacksmith will help you out with this. (12.) A center-punch or sharp-pointed punch for making dents in metal. A sharp-pointed wire nail will do for tin and copper. (13.) Files for metal. (14.) Some sort of a vice or clamp. (See App. 79, 80.) (15.) Shears for cutting sheet-tin, etc. A pair of old shears will do. (16.) An anvil or piece of old iron that may be used to hammer on to flatten tin, etc. An old flat-iron makes a good anvil. (17.) Hammer.
The small hollow handle tool sets are very handy, and they contain small chisels, awls, screw-driver, etc. These sets cost from 50 cents up.
284. Materials. For wood you will find the sides and ends of clean soap or starch boxes about the right thickness; they are fairly smooth to begin with. For thin wood use cigar boxes. The pieces from old boxes should be removed with care, and saved in one place, which may be called your lumber yard. All nails should be removed with a claw-hammer. Look out for nails when using a saw, plane or other edged tool. (See § 297.) The edges of bases, etc., may be bevelled as shown in Fig. 95. This is not necessary, but it adds greatly to the appearance.
285. Screw-Eyes. Brass screw-eyes, with copper burs, make excellent binding-posts. (App. 45, 46.) Those that are ⅜ in. in diameter inside the circle are about right. These are about 1¼ in. long in all, with a ½ in. thread.
286. Copper Burs, such as are used with rivets, are very handy. The size that is ½ in. in diameter, with a ⅛ in. hole, is good.
287. Copper Wire. This can be bought at an electrician's. The only trouble, however, in buying small quantities is that you may have to pay a large price in proportion. If you get it on ½ lb. spools you can handle it much better (see App. 23) than you can if you have it in a tangle. It is well to have ½ lb. of No. 24 or 25 for electro-magnets, current-detectors, etc., etc. ½ lb. of No. 30 will not be too much, if you make induction coils. If you handle your wire carefully, single cotton-covered will do. Double cotton-covered is better than single, but it costs more. Be careful not to injure the covering. (See below for splicing wire.) Look out for broken wire.
288. Splicing Wire. Fig. 125. Do not simply touch two wires together and imagine that you have a good connection; a mere twist is not sufficient. Clean the ends of old wire thoroughly with a file or knife-blade, and join them as shown in Fig. 125.
289. Copper. Sheet-copper can be purchased at a tinsmith's or at a hardware store. Electricians usually have a thin variety of copper called brush copper, which makes good battery-plates, binding-posts, etc. You can cut this thin copper with an ordinary pair of shears.
290. Iron. For thin sheet-iron, nothing is better than sheet-tin. (See tin.) Hoop iron is thicker than tin, and makes good yokes, etc. In many cases, ordinary nails may be used where a magnetic substance is needed. Annealed iron wire is extremely soft. (See text-book for experiments with steel and iron.)
291. Steel. Old files, watch-springs, clock-springs, corset-steels, knitting-needles, harness-needles, hack-saw blades, sewing-needles, etc., are generally made of a good quality of steel.
292. Zinc, in the sheet form, can be bought at a hardware store. For a few cents you can get quite a large piece. Get the thick pieces for heavy battery-plates of an electrician. You do not need anything that is thicker than ⅛ in. The zinc rods are usually amalgamated.
293. Lead can be bought at a plumber's, tinsmith's, or hardware store. You may want some for a storage cell.
294. Nails. Wire nails are best for light work. Get an assortment from ½ in. long up to 1½ in.
295. Screws. It is better to use brass screws around electrical apparatus. For the small work, for binding-posts, etc., use ⅝ No. 5. Another handy size is No. 7, from ¾ to 1¼ in. long. The round-headed screws are best, unless you want to countersink them.
296. Tin. This is really thin sheet-iron, covered with tin. Save up tomato-cans, cracker-boxes, condensed-milk cans, etc. The cracker-boxes are just as good as sheet-tin, as the pieces are large and clean. You can remove the solder from cans by heating them in the kitchen fire. Knock out the bottoms with a poker when the solder gets soft. Clean the tin with sand-paper.
297. Carbons. You can get carbon rods or plates at an electrician's. If you have arc electric lights in your city, you will be able to pick up carbons; these, however, generally have a coating of copper, which must be eaten off with dilute nitric acid. This is a bother. You will find it cheaper to buy the ½ in. rods that are 12 in. long, and uncoated.
298. Shellac. Your wood-work will be much improved by using shellac upon it after you have thoroughly sand-papered it. You can get it, all prepared, at a paint store. Wood-alcohol is used to thin it if it gets too thick. Keep it in a wide-mouth bottle. Paint it on quickly and evenly with a brush, and do not go over it again when it is partly dry. Wait until it is thoroughly hard before putting on a second coat. It should be fairly thin to spread well. Clean your brush in wood-alcohol before putting it away, and keep the shellac bottle tightly corked. A small tin can or a teacup is best to hold the shellac when using it.
HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS
By THOMAS M. ST. JOHN, Met. E.