For the switch-bar use a piece of hard copper or brass four inches long, half an inch wide, and about one-eighth of an inch thick, or the same thickness as the copper straps at the ends of the mica fuse-plate. Bore a hole at one end of this bar, and with a copper rivet attach it between the two upright ears at the middle of the block. With a file cut away the two edges at the other end of the bar for a distance of an inch, so that the bar will have an end as shown at C in Fig. 26. Drive a small file-handle on this end and give it a coat or two of shellac; then bevel the lower edges of the bar with a file where it enters between the blades of the clutch. The circuit wires are made fast at both ends of the block, and the current travels through the binding-posts, the lead fuse-wire on the mica plate D, and the switch-bar. If the current is too strong, then when the switch-bar is pushed into the clutch the safety-fuse will burn out and save the apparatus; or it will arrest a flash of lightning.
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.