Part 17 (1/2)

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 pa.s.s 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 pa.s.s 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 b.i.+.c.hromate cells, like App. 3 or 4.

270. Operation. The current will pa.s.s 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 1/2 revolution. (See text-book for full explanations and for simple experiments with electric motors.)

APPARATUS 147.

_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 1/4 in. one put in instead.

This may be done by making a handle for the 1/4 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.

CHAPTER XX.

ODDS AND ENDS.

APPARATUS 148.

_272. Graduated Circles._ Fig. 123. For compa.s.ses (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.

[Ill.u.s.tration: Fig. 123.]

To divide the circle. Divide the circ.u.mference 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.

APPARATUS 149.

_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 bra.s.s screws, not with nails, as these would affect the needle. It is placed at one side of D so that the compa.s.s 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.

[Ill.u.s.tration: Fig. 124.]

APPARATUS 150.

_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 1/4 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.

APPARATUS 151.

_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 pa.s.sed 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 pa.s.s 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.

APPARATUS 152.