Harper's Electricity Book for Boys

by Joseph H. Adams

Simple and Horseshoe Magnets

Every boy has a horseshoe magnet among his collection of useful odds and ends, and whether it is a large or small one its working principle is the same. If large enough it will lift a jack-knife, nails, or solid weights, such as a small flat-iron or an iron padlock. A horseshoe magnet is made of highly tempered steel and magnetized so that one end is a north pole and the other a south pole. In more scientific language these poles are known as, respectively, positive and negative. Once magnetized the instrument retains that property indefinitely, unless the power is drawn from it by exposure to intense heat, and even then, by successive heating and cooling, the magnetism may be partially restored.

An electro-magnet may be made of any scrap of soft iron, from a piece of ordinary telegraph-wire to a gigantic iron shaft. When a current of electricity passes through a wire a magnetic “field” is produced around the wire, and if the latter is insulated with a covering and coiled about a soft iron object, such as a nail, a bolt, or a rod, that object becomes a magnet so long as a current of electricity is passing through the coils of wire or helix. A coil of wire in the form of a spiral spring has a stronger field than a straight wire carrying the same current, for each turn or convolution adds its magnetic field to that of the other turns.

A simple form of electro-magnet is made by winding several layers of No. 20 insulated copper wire around a stout nail or a carriage-bolt; by connecting the ends to a battery of sufficient power, some very heavy objects may be lifted. A single magnet, like the one shown in [Fig. 1], is made with a piece of soft iron rod six inches long and half an inch in diameter, the ends of a large spool sawed off and worked on the rod, and half a pound of No. 20 insulated copper wire. The spool-ends are arranged as shown in [Fig. 2]. An end of the wire is passed through a hole in one flange when you begin to wind the coils, and when finished, the other end is passed through a hole at the outer rim of the same flange. This magnet may be held in the hands when in use; or a hand-magnet may be constructed of a longer piece of iron on one end of which a handle is driven and held in place with a nut and washer, as shown in [Fig. 3]. The wires from the coil pass through holes made in the handle and come out at the butt end, where they may be attached by connectors to the pole-wires of a battery. To protect the outer insulated coil of wire from chafing and a possible short-circuit, it would be well to wrap several thicknesses of stout paper around the coil and glue it fast; or a leather cover will answer as well.

SIMPLE AND HORSESHOE MAGNETS

A more powerful magnet may be made from a stout bolt, two nuts, and a wooden base, with about three-quarters of a pound of No. 18 insulated copper wire to wind about the body of the bolt. A block of wood an inch thick, four inches wide, and six inches long is provided with a hole at the middle for the bolt to pass through. A larger hole is made at the under side of the block so that a nut can be easily turned in it. A three-quarter-inch machine-bolt, with a square head, and seven inches long, is set in the block, head up, as shown in [Fig. 4]; and composition or thin wooden disks or washers are placed on the bolt to hold the coils of wire in place. The ends of the wire pass out through the bottom washer and are made fast to binding-posts on the block, and to these latter the battery-poles are made fast when the magnet is in use. Coils of wire may be wound on an ordinary spool, and the hole in the middle may be filled with lengths of soft iron wire. When a current is passing around the spool the wires become highly magnetic, but lose the magnetism directly the current ceases.

Horseshoe electro-magnets are made by winding coils on the ends of U-shaped pieces of soft iron, but the winding must be done so that the current will pass around them in opposite directions, otherwise you would have two negatives instead of a negative and positive. For a small horseshoe magnet a stout iron staple may be used, but for the larger magnets it would be best to have a blacksmith bend a piece of round iron in the desired shape.

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.