The principal objection to the ordinary receiver is that it does not contain enough turns of wire on its bobbins. This is easily remedied by carefully rewinding them with a very fine silk covered, pure copper magnet wire no larger than No. 40 B. S. gauge. This will increase the number of turns and also the resistance, but it must not be inferred that resistance is to be desired. This is a common impression of amateurs who do not understand the underlying principle, that the strength of an electromagnet varies directly as the number of turns of wire, multiplied by the amperes flowing through the magnet. When a telephone receiver is wound with a finer wire the resistance is increased, cutting down both the current and the strength of the magnet. But if pure copper wire is used, and the winding not carried beyond the point where the circumference of the outside layer becomes twice as great as the circumference of the first layer, the number of turns increases faster than the resistance and the magnet strength is considerably greater than before the receiver was rewound.
No. 40 B. S. gauge silk covered wire is often used for this purpose, but the best results are obtained with enameled covered wire of the same size. It is possible to wind almost three times as much of the enameled wire on a telephone bobbin as silk wire of the same size. The difference is due to the thickness of the insulation. An ordinary double pole watch-case receiver will have a resistance of 800-1,000 ohms when wound with silk covered wire and 1,500-1,800 ohms when wound with enameled wire.
To rewind a telephone receiver, first unscrew the cap and remove the diaphragm, then remove the bobbins by unloosening the screws with the aid of a screw driver. Unwind the old wire and examine the empty bobbins to see that wherever the wire is liable to come into contact with the metal that it is well insulated with paraffined paper or some other equally good material. Then wind the new wire on in smooth even layers and when it is completed fasten the bobbins back on the permanent magnets and connect them up. The current should flow through in opposite directions so that the north pole of one and the south pole of the other is on top. Do not trust splice connections but solder them using acid as a flux.