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 bichromate 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.

_277. Battery Jars._ For small cells, use gla.s.s tumblers. Ordinary gla.s.s 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. Gla.s.s 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 k.n.o.b 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 1/8 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 gla.s.s tubing, rubber tubing, etc., in your shop. Draw out the gla.s.s so that the gas will burn in a fine needle-like flame about 1 in. long. Keep the point of the flame about 1/4 in. ahead of the crack. The gla.s.s tube should be held in a rubber tube connected with the gas pipe.

CHAPTER XXI.

TOOLS AND MATERIALS.

_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 subst.i.tutes 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 1/8 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 coa.r.s.e 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 gla.s.s, 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, 1/8 in. in diameter. You should have one 1/4 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. Bra.s.s screw-eyes, with copper burs, make excellent binding-posts. (App. 45, 46.) Those that are 3/8 in. in diameter inside the circle are about right. These are about 1-1/4 in. long in all, with a 1/2 in. thread.

286. Copper Burs, such as are used with rivets, are very handy. The size that is 1/2 in. in diameter, with a 1/8 in. hole, is good.

[Ill.u.s.tration: Fig. 125.]

287. Copper Wire. This can be bought at an electrician's. The only trouble, however, in buying small quant.i.ties is that you may have to pay a large price in proportion. If you get it on 1/2 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 1/2 lb. of No. 24 or 25 for electro-magnets, current-detectors, etc., etc. 1/2 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.