Part 16 (1/2)

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 tue of the box (See experi it with a charged body The thread will shohether the charge resides upon the inside or upon the outside of the box

APPARATUS 143

[Illustration: Fig 115]

_248 Support for Electrified Co 115 In the study of static electricity, ordinary ebonite coe 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 ofthem All that can be done here, however, is to sho to make simple motors (See text-book for experi 116, 117 Fig 116 shows a plan or top view, and Fig 117 shows a side vieith 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-nets on a motor are called, areThe 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 screever, 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 fro

116)

252 The Armature, A, isand 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 h 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 for 116]

253 The shaft or axle, 8, isand 1/4 in in diath, 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 ulated as to position

[Illustration: Fig 117]

255 The Co 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 cos 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 coh 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 dentno 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