A telephone receiver that will do good work may be built very cheaply as follows: For the case use an ordinary 1/2-lb. baking-powder box with a piece of heavy wire soldered on the inside, 1-5/16 in. from the bottom. For the magnet use a piece of round hardened steel about 3/8 in. in diameter and 1-1/4 in. long. If desired, a piece of an old round file may be used for the magnet core, which should be magnetized previous to assembling, either by passing a current of electricity around it, or by direct contact with another magnet. The steel core should be wound with about 250 ft. of No. 36 insulated wire, the ends of which should be soldered to a piece of lamp cord, passed through a hole in the bottom of the can and knotted inside to prevent pulling out.
A disk of thin sheet-iron, such as is used by photographers for tintypes (Ferrotype), should be cut to the diameter of the can, taking care not to bend the iron. The magnet should then be placed in the bottom of the can in an upright position and enough of a melted mixture of beeswax and resin poured in to hold it in position.
While the wax is still in a plastic condition the magnet should be located centrally and adjusted so that the end will be 1/16 in. or less below the level of the top of the copper ring.
After the wax has hardened the disk is slipped in and fastened tightly by a ring of solder when the instrument is ready for use.