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. [Illustration: Fig. 237.—A Simple Arrangement showing the Inductive Action between two Coils.] 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. [Illustration: 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.