Figs. 177-179.—The Electro-magnet.
You will need a machine-bolt or carriage-bolt 2½ or 3 inches long, and ¼ inch in diameter, for the core of the magnet, some insulated electric-bell wire for the coil, and a piece of heavy cardboard. Cut three washers of a trifle larger diameter than the bolt-head, out of the piece of cardboard ([Fig. 178]), and slip these over the bolt as shown in [Fig. 179]—one at the bolt-head end, the other two at the nut end; then screw the nut on to the end of the bolt.
Fig. 180.—How the Electro-magnet is Connected up.
Before starting to wind the insulated wire upon the bolt, pierce two holes through the inner cardboard washer of the two at the nut end. Then stick the end of the wire through one of these holes, and pull a length of 4 or 5 inches of the wire out between the two washers. Starting at this end of the bolt, then, wind the wire around the bolt, keeping the turns even and each turn pressed close against the preceding turn. When the washer at the head end of the bolt has been reached, wind back to the starting point; then wind back to the washer at the head a second time, and again back to the starting point; and so on until six or eight layers of wire have been wound in place. An even number of layers will bring the free end of the wire back to the double-washer end. Slip this end through the second hole in the inner washer, and bring it out between the two washers, as you did the first end. Then screw the bolt-nut tight against the washers, to hold the wire ends in place ([Fig. 177]). The outer cardboard washer will prevent the nut from chafing the insulation on the wire ends.
Now connect the ends of the coil to the binding-posts of a battery cell, and you will be surprised to find what a strong magnet the head of the bolt core has become.
One end of the magnet coil should be connected to a dry-cell, and the other to a switch; and another wire should connect the switch with the dry-cell ([Fig. 180]).
A Home-made Switch that is easily made is shown in [Fig. 181]. Cut strips A, B, and C ([Fig. 182]) from a tomato can. Tack the turned up ends of A to a wooden knob (D). This forms the switch lever. Strips B and C, folded in half, and punched near the ends, form the binding-post plates.