How Two Boys Made Their Own Electrical Apparatus Part 16

APPARATUS 142.

_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.

APPARATUS 143.

[Ill.u.s.tration: Fig. 115.]

_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.

CHAPTER XIX.

ELECTRIC MOTORS.

_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.)

APPARATUS 144.

_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 7/8 in. The upright, U, is 3-1/2 1-1/2 1/2 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, 2-1/2 in. long. The washers are 1-1/2 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 1-1/2 in. from the back edge of B. (Fig.

116).

252. The Armature, A, is made of 6 or 8 thicknesses of tin, 2-1/2 in.

long and 3/4 wide. (See App. 71.) In its center is punched or drilled a 1/4 in. hole, so that it can be slipped onto the 1/4 in. "sink-bolt," 8.

If you have taps you can make the hole a little smaller than 1/4 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.)

[Ill.u.s.tration: Fig. 116.]

253. The shaft or axle, 8, is made of a "sink-bolt" that is 3 in. long and 1/4 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 3/4 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.

[Ill.u.s.tration: Fig. 117.]

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 sc.r.a.pes 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 sc.r.a.pe 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 pa.s.ses 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 gla.s.s, 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 pa.s.s, 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 pa.s.s again, until A is at least at right angles to a line drawn through the center of the coils.

If the current again pa.s.ses, the ends of A will be attracted by the bolt-cores.

In other words, the current should pa.s.s 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 1/4 of a revolution without current, and just after it gets beyond this, the current pa.s.ses for nearly 1/4 of a revolution, which brings the ends over the poles again. The next 1/4 of a turn it has no current, because the flat side of 9 is opposite the brush, 10, as during the first 1/4.

The last 1/4 the current pa.s.ses 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.

[Ill.u.s.tration: Fig. 118.]

APPARATUS 145.

_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, 1-1/2 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 pa.s.ses through E, and holds the piece of 1/4 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 1/2 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 1/8 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).

APPARATUS 146.

_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 7/8 in. The uprights, U, are 3-1/2 1 3/4 in. They are screwed or nailed to the base from below, their 1-in. sides being towards you in Fig. 119. They are 4-1/4 in. apart, inside, in this model. The piece, A, is 2-1/2 7/8 5/8 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 6-1/4 3/4. In its ends are slots, and in its center is a hole so that the 1/4 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.

[Ill.u.s.tration: Fig. 119.]

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 1-1/8 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 1/4 in. stove-bolts, 1-1/4 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 1/4 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 5/8 in. apart, inside. The cores are wound with 4 or 6 layers of No. 24 or 25 wire. This makes them about 3/4 in. in diameter. They are held in a tin yoke, 14, made of 5 or 6 thicknesses of tin. 14 is 3 3/4 in., and has 3 holes punched in it. The two outside holes are 2 in. apart. Through these pa.s.s the bolts, which are held firmly by the 2 nuts. The shaft, S B, is a sink-bolt, 3 in. long, and 1/4 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.

[Ill.u.s.tration: Fig. 120.]

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 3/8 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.

[Ill.u.s.tration: Fig. 121.]

[Ill.u.s.tration: Fig. 122.]

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 pa.s.s 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 1/8 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 1-1/4 3/4 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.