A powerful horseshoe magnet may be made from a piece of tire-iron bent as shown in Fig. 5 A; when wound with No. 18 wire it will appear like Fig. 5 B. A volt or two of current passing through the coils will render this magnet powerful enough to lift several pounds. 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.