A coherer set of this sort may be used on an aerial and ground by substituting the coherer for the detector, but otherwise following any of the receiving circuits which have already been shown.
CHAPTER XV A WIRELESS TELEPHONE
Probably many readers of the "Boy Electrician" are amateur wireless operators and have constructed their own apparatus with which they are able to pick up commercial messages or communicate with other experimenters in the neighborhood, but not many have ever built a wireless telephone.
The device described in the following pages is easy to make and arrange, and will serve for some very interesting experiments.
It is of no practical value as a commercial wireless telephone, because the distance over which it will transmit speech is limited to from 250 to 300 feet. If you have a chum who lives across the street and within the distance named above, it is possible for you to construct a simple wireless telephone which will enable you to remain in your own rooms and talk to each other without any connecting wires.
The instruments operate by magnetic induction. It has already been explained how it is possible for the current in the primary of an induction coil to induce a current in the secondary coil, even though the two are not electrically connected. This type of wireless telephone really consists of an induction coil in which the two windings are widely separated.
Suppose that two coils of wire are connected as in Figure 237. The illustration shows that one coil, A, is connected in series with a set of batteries and a telegraph key. The terminals of the other coil, B, are connected to a telephone receiver. The coils are placed parallel to each other and a few inches apart. If the key is pressed so that the battery current may flow through the coil, A, it will create a magnetic field, and lines of force will be set up in the immediate vicinity. The lines of force will pass through the coil, B, and induce in it a current of electricity which will cause a sound like a click to be heard in the telephone receiver.
Fig. 237.—A Simple Arrangement showing the Inductive Action between two Coils.