It is made from a piece of curtain-pole one inch and an eighth in diameter and three inches and a half long. A hole three-eighths of an inch in diameter is bored its entire length at the middle, and through this the magnet passes. At one end of this tube a wooden pill-box (E) is made fast with glue, or a wooden cup may be turned out on a lathe and attached to the magnet tube. If the pill-box is employed it should be two inches and a half in diameter, and at four equidistant places inside the box small lugs of wood are to be glued fast. Into these lugs the screws employed to hold the cap are driven. The walls of pill-boxes are so thin that without these lugs the cap could not be fastened over the thin disk of metal (D) unless it were tied or wired on, and that would not look well. If the cup is turned the walls should be left thick enough to pass the screws into, and the inside diameter should then be one inch and three-quarters.
The cap (B) is made from thin wood, fibre, or hard rubber. It is provided with a thin rim or collar to separate its inner side from the face of the disk (D). Four small holes are bored near the edge of this cap, so that the screws which hold it fast to the cup (E) may pass through them. The magnet (M) is a piece of hard steel three-eighths of an inch in diameter and four inches and a quarter long. This may be purchased at a supply-house, and if it is not hard enough a blacksmith can make it so by heating and plunging it in cold water several times. It may be magnetized by rubbing it over the surface of a large horseshoe magnet, or if you live near a power station you can get one of the workmen to magnetize it for you at a trifling cost. Should you happen to possess a bar magnet of soft iron with a number of coils of wire, and also a storage-battery, the steel bar may be substituted for the soft iron core and the current turned on. After five minutes the steel can be withdrawn. It is now a magnet, and will hold its magnetism indefinitely.
Now have a thin, flat spool turned from maple or boxwood to fit over one end of the rod, and wind it with a number of layers of No. 36 copper wire insulated with silk. This is known in the electrical supply-houses as “phone”-receiver insulated wire, and will cost about fifty cents an ounce. One ounce will be enough for two receivers. It should be wound evenly and smoothly, like the strands of thread on a spool, and this may be done with the aid of the winder described on page 58.