If a telephone transmitter is substituted for the key and words are spoken into it, the current passing through the coil from the battery will vary with each vibration of the voice and the words will be distinctly repeated by the receiver connected to B.
This experiment may be tried by any boy with the equipment he probably has already around his shop. Twenty-five to thirty turns of wire wound around a cardboard tube five or six inches in diameter will serve as a coil. Two such coils, an ordinary telephone transmitter, a telephone receiver and a couple of dry cells are all that is required.
Fig. 238.—A Simple Wireless Telephone. Speech directed into the Transmitter can be heard in the Receiver, although there is no direct electrical connection between the two.
The diagram in the accompanying illustration shows how the apparatus is arranged. The coils may be used several inches apart and the voice will be clearly heard in the receiver.
Such an outfit is, however, only experimental, and if it is desired to make a practical set, the coils, etc., must be much larger in diameter and contain a greater number of turns.
Larger coils are made by first drawing a circle four feet in diameter on the floor of the "shop" or attic. Then drive a number of small nails around the circumference, about four inches apart.
Procure two and one-half pounds of No. 20 B. & S. gauge cotton-covered magnet wire and wind it around the circumference of the circle. The wire should form at least sixty complete turns. About one foot should be left at each end to establish connections with. After winding, the coil should be tied about every six inches with a small piece of string so that it will retain its shape and not come apart. The nails are then pulled out so that the coil may be removed.
The coil may be used just as it is for experimental purposes, but if it is intended for any amount of handling it is wise to procure a large hoop such as girls use for rolling along the sidewalk, and make the coil the same diameter as the hoop so that upon completion they may be firmly bound together with some insulating tape. Two binding-posts may then be fastened to the hoop and the terminals of the coil connected to them.
Two such coils are required for a complete wireless telephone system, one to be located at each station.