The Boy Electrician

by Alfred Powell Morgan

Fig. 30.—Method of Finding the Center of a Circle.

Select a smooth bottle which has no lettering embossed upon it, and stand it upon a piece of white paper. Trace on the paper a line around the circumference of the bottle so that the circle thus formed is of the same size as the bottom of the bottle. Lay a carpenter’s square on the circle so that the point C just touches the circumference. Draw a line from A to B where the sides of the square cut the circumference. The point in the middle of this line is the center of the circle.

Place the paper on the bottom of the bottle so that the circle coincides with the circumference, and mark the center of the bottle.

The bottle must now be drilled. This is accomplished with a small three-cornered file, the end of which has been broken off so as to form a ragged cutting edge. The file is set in a brace and used like an ordinary drill. During the boring process the drill must be frequently lubricated with a mixture of gum camphor and turpentine. The drilling, which will require almost an hour before the glass is pierced, if the bottle is a thick one, should be performed slowly and carefully, so as to avoid all danger of cracking the glass. The hole, when finished, should be from one-quarter to three-eighths of an inch in diameter.

After the hole has been bored, fit a wooden plug into the neck of the bottle and cement it there with a mixture composed of one-half a pound of resin, five ounces of beeswax, one-quarter of an ounce of plaster of Paris, and three-quarters of an ounce of red ocher, melted together over a moderately warm stove. Dip the plug in the molten cement and force it into the neck of the bottle. When the cement dries it will be impossible to remove it.

The sizes of bottles vary, so that it is quite impossible to give dimensions which must be closely followed in constructing the machine. Those in the text are approximate. The drawings have been made to scale so as to show the proportions the parts bear to each other.

A heavy wooden base will be required to mount the machine on. Two uprights are mounted on the base to support the axis of the bottle. Through one of these bore a hole of the same diameter as the wooden plug fitted in the neck of the bottle. The end of the wooden plug projecting through the upright is notched and fitted with a crank so that the bottle may be revolved. The handle of the crank is an ordinary spool having one flange cut off and mounted with a screw and a washer.

Fig. 31.—The "Rubber."

The machine is now ready for the "rubber" and "prime conductor." The rubber is a piece of wood one inch square and from six to eight inches long. A piece of undressed leather is tacked on as shown in the illustration and stuffed with horsehair. The wood is shellacked and covered with tin-foil previous to tacking on the leather. A strip of wood, two inches wide and one-half an inch thick, is fastened to the back of the rubber. The strip should be just long enough so that when the lower end rests on the base the rubber is level with the axis of the bottle. The lower end may be fastened to the base by means of a small brass hinge. Two rubber bands stretch from hooks between the rubber and the base so as to pull the former tightly against the bottle. The illustration shows a method of mounting the rubber on a foot-piece held to the base with a thumb-nut so that it may be slid back and forth and the pressure varied at will.