You are now likely to find that if you press the key of the transmitter, the decoherer will ring. It is possible that it will continue to ring after you have stopped pressing the key. If such is the case, it will be necessary to turn the adjusting screw on the relay so as to move the armature upward a short distance away from the core. If the decoherer will not operate each time when you press the key, the brass plugs in the coherer need adjusting. You must not be discouraged if you have some difficulty in making the apparatus work at first. After you learn how to adjust it properly, you will find that you can move the transmitter quite a distance away from the coherer and it will still operate very nicely. After you manage that, you can place the apparatus in separate rooms and find it possible to work it just the same, because ordinary walls will not make any difference to wireless waves. Bear in mind that the nearer the coherer plugs are to each other, the more sensitive the coherer will be, but that if too close, the decoherer will not be able to shake the filings properly and will not stop when you stop pressing the key. The operation of the apparatus depends upon the fact that when properly adjusted the resistance of the filings between the two brass plugs is too great to allow sufficient battery current to flow to attract the armature of the relay. As soon as any wireless waves from the transmitter strike the catch-wires of the coherer, they cause the filings to cling together or cohere. When in this state, they have a low resistance and permit the current to flow in the relay circuit and draw down the armature. The armature closes the second circuit and sets the decoherer into operation. The decoherer shakes the filings and causes them to decohere or fall apart and so makes them ready again for the next signal. 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.