One of the first pieces of apparatus the amateur electrician desires to make is an electro-magnet. As some of the materials generally used are not obtainable except in the large cities, resort must be had to substitutes, of which those mentioned below can be obtained in every place in which a blacksmith shop is to be found.
For the cores the softest iron is necessary. Norway bar iron is the best kind to use, and in buying it be sure that it is iron and not a low carbon steel, as much of the so-called iron now sold is in reality steel and not iron. If Norway iron cannot be had, the next best is to apply to the blacksmith shop for some rivets of the required size. To ensure that these are as soft as possible they should be annealed by heating to a red heat in a stove or furnace, and then allowing them to cool slowly by packing in ashes.
A yoke is made from a piece of flat bar iron, holes being drilled of the right size to receive the rivets with a, drive fit, and spaced the proper distance apart to allow of a slight space between the coils. Each coil must be insulated from the core. This is done by winding several turns of paper around a round piece of wood, which has been previously covered with soap, giving •each turn of paper a coating of thin shellac. The soap prevents the shellac from attaching the paper to the wood. At each end of the paper tube thus formed attach with shellac a piece of thin, round or square wood with a hole in the center just fitting the tube. Suitable pieces can be cut from a piece of cigar box wood. These ends are also coated with shellac, and the coil bobbins are complete.
One bobbin, with the wooden rod still remaining therein, is then mounted on an empty box, on the edges of which notches have been cut to hold the rod in position. A piece of wire bent to form a crank handle is then added to one end of the rod and the wire then wound on in even layers. Unless currents of considerable potential are to be used, no insulation of other than the wire covering will be be needed, thus allowing the wire to be removed for other uses, should the maker ever desire. To start the winding, bore a small hole in one of the ends, close to the paper tube. Put through the hole from the inside of the wire and then, slowly turning the handle, wind on the wire in even layers, back and forth, until the coil is wound, carrying the outer end through another hole bored in the other end piece at the proper place. The other coil is made in the same way, with the exception that the wire is wound in the opposite direction.
The two coils can now be placed on the cores, first coating the latter with shellac, which will hold the coils firmly in place. The ends of the coils having the inside ends of the windings should be placed next to the yoke, and these ends connected. The outer ends of the windings can then be connected with the current supply, and the magnet is in working order. For magnets which are to be used for simple experiments, No 20 to 24 gauge wire is about right; larger wire uses up the battery too rapidly.