The Boy Electrician

by Alfred Powell Morgan

Fig. 236.—The Complete Coherer Outfit.

If you wish to use the outfit for demonstration purposes or for sending messages for very short distances, as for instance across a room, you do not need an aerial but merely a pair of "catch-wires."

The "catch-wires" are two pieces of stiff copper wire, about two feet long, placed in the lower holes in the double binding-posts forming part of the coherer.

In order to set the apparatus for operation, raise the adjusting screw of the relay until the armature is quite far away from the core. Then push the armature down against the contact on the core. The decoherer should then immediately operate and begin to tap the coherer. Then turn the thumbscrew until the armature is brought down to the core in such a position that it is as close as it is possible to get it without ringing the bell.

The transmitter should consist of a spark coil, battery, key, and a spark-gap. The gap should be connected to the secondary of the coil and adjusted so that the electrodes are only about one-eighth of an inch apart. The key is placed in series with the primary of the coil and the battery, so that pressing the key will send a stream of sparks across the gap. Fit the spark-gap with two catch-wires similar to those on the coherer and place the transmitter about four or five feet away from the coherer outfit.

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.