CHAPTER XX.

ODDS AND ENDS.

APPARATUS 148.

_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.

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 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.

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 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 Sec. 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.

APPARATUS 152.

_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 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 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 1/4 in. ahead of the crack. The glass tube should be held in a rubber tube connected with the gas pipe.