Harper's Electricity Book for Boys

by Joseph H. Adams

For bells, telegraph-sounders, and other electrical equipment requiring the horseshoe or double magnet, several kinds may be used, but the simplest is constructed from two carriage or machine bolts and a yoke of soft iron, as shown in [Fig. 6]. The yoke is five-eighths of an inch in width, two inches and a half long, and provided with two three-eighths-inch holes, one inch and a half apart from centre to centre. Two-inch carriage or machine bolts are used, and they should be three-eighths of an inch in diameter. The nuts are turned on the thread far enough to admit the yoke, and then another nut is applied to hold it in place and bind the three pieces into one compact mass. Wooden spool-ends or composition washers are placed on the bolts to hold the ends of the wire coils in place, and the winding may be done on each bolt separately and locked to the yoke after the winding is completed. Double cotton-insulated No. 20 or 22 copper wire should be used for the coils.

It is a tedious and bothersome job to wind a coil by hand, and if possible a winder should be employed for this purpose. Several varieties of winders are on the market, but a simple one for ordinary spools may be made from a stick held in an upright piece of wood with staples. This idea is pictured in [Fig. 7], where the round stick is shown cut with two grooves into which the staples fit. One end of the stick is made with a square shoulder, so that a handle and crank can be fitted to it. A few wraps of wire are taken around the crank to prevent it from splitting, and it is held to the round stick with a slim steel nail. The opposite end of the round stick is shaved off so that it will fit snugly in the hole of a spool; if it should be too small for some spools, a few turns of cord around the small end will make it bind. The block to which the shaft and crank is attached may be held in a vise or screwed to the edge of a table.

Induction-coils

A simple induction or shocking coil may be made of a two-and-one-half by five-sixteenths-inch bolt, a thin wooden spool, and two sizes of insulated copper wire. An induction-coil is a peculiar and wonderful apparatus; it figures largely in electrical experimenting and is a part of every complete equipment.

A piece of curtain-pole may be used for the spool. First bore a five-sixteenths-inch hole through the wood to receive the bolt; then in a lathe turn it down into a spool with less than one-eighth of an inch of wood about the hole and with flanges about one-eighth of an inch in thickness. Smooth the spool with sand-paper, while it is still in the lathe, and give it a thin coat or two of shellac.

Slip the spool on the winder ([Fig. 7]) and wind on three layers of No. 24 cotton-insulated copper wire, taking care to wrap the coils evenly and close. Bring six inches of the ends out at either end of the spool through small holes pierced in the flanges; then wrap several thicknesses of brown paper around the coil. A current passing around this three-layer coil will magnetize the bolt. This is the primary coil and the one through which the battery current will pass.

A secondary coil is now made over the primary one with eleven or thirteen layers of No. 30 insulated copper wire. It will take some time to carefully put on these layers, and when doing so mark down each layer so as to keep an accurate count, for there must be the right number of layers to make the coil act properly. No. 30 wire is quite fine, and if the layers are not inclined to lie smooth, make a wrap or two of brown paper between each three layers. Bring six inches of each end of the wire out from the flanges of the spool, and to protect the outer coil wrap paper about the coils and attach it fast with thread or paraffine. Slip the bolt through the hole and screw the nut on the threaded end. Cut out a wooden block four inches long, three inches wide, and three-quarters of an inch thick, and with two thin metal straps and screws attach the coil to the middle of the block, as shown in [Fig. 8]. Make four binding-posts and screw them fast at the corners, and to A and B of [Fig. 8] attach the ends of the heavy wire from the primary coil, and to C and D of [Fig. 8] the ends of the fine wire from the secondary coils. The induction-coil is now ready for any use to which it may be put, and by mounting it on the block with the delicate wire ends attached to the binding-posts, it is in less danger of damage than if the wire ends were left loose for rough-and-ready connections.

In order to get a shock from this coil it will be necessary to have a pair of handles and a current interrupter. The handles may be made from two pieces of tin rolled into the form of cylinders to which wires are soldered. Or, better yet, use pieces of thin brass tubing an inch in diameter. The buzzer shown in [Fig. 9] may be employed for a current interrupter, and a bichromate battery will furnish the current.

In order to make the connections, the wires from the handles are attached to the binding-posts C and D in [Fig. 8]—that is, to the wires of the secondary coil. One spool of the battery is connected with A of [Fig. 8] and the other with A of [Fig. 9]. A wire connects C of [Fig. 9] with B of [Fig. 8], and the circuit is closed. The buzzer now begins to vibrate, and any one holding the handles will receive a shock the intensity of which depends on the strength of the batteries. A switch should be introduced somewhere in the circuit, so that it may be opened or closed at will; a good place for it is between a battery-pole and the binding-post A in [Fig. 8].

If the shock is too intense it may be weakened by drawing the carbon and zinc poles partly out of the bichromate solution; or a regulator may be made of a large glass tube and a glass preserving-jar filled with water. If the tube cannot be had, an Argand gas-burner chimney will answer as well.