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| original © Underwood and Underwood. General Pershing Listening In. |
The Crystal Detector.--A crystal detector consists of: (1) the frame, (2) the crystal, and (3) the wire point. There are any number of different designs for frames, the idea being to provide a device that will (a) hold the sensitive crystal firmly in place, and yet permit of its removal, (b) to permit the wire point, or electrode, to be moved in any direction so that the free point of it can make contact with the most sensitive spot on the crystal and (c) to vary the pressure of the wire on the crystal.
A simple detector frame is shown in the cross-section at A in Fig. 10; the crystal, which may be galena, silicon or iron pyrites, is held securely in a holder while the phosphor-bronze wire point which makes contact with it, is fixed to one end of a threaded rod on the other end of which is a knob. This rod screws into and through a sleeve fixed to a ball that sets between two brass standards and this permits an up and down or a side to side adjustment of the metal point while the pressure of it on the crystal is regulated by the screw.
A crystal of this kind is often enclosed in a glass cylinder and this makes it retain its sensitiveness for a much longer time than if it were exposed to dust and moisture. An upright type of this detector can be bought for $2.25, while a horizontal type, as shown at B, can be bought for $2.75. Galena is the crystal that is generally used, for, while it is not quite as sensitive as silicon and iron pyrites, it is easier to obtain a sensitive piece.
The Tuning Coil.--It is with the tuning coil that you tune in and tune out different stations and this you do by sliding the contacts to and fro over the turns of wire; in this way you vary the inductance and capacitance, that is, the constants of the receiving circuits and so make them receive electric waves, that is, wireless waves, of different lengths.
The Double Slide Tuning Coil.--With this tuning coil you can receive waves from any station up to 1,000 meters in length. One of the ends of the coil of wire connects with the binding post marked a in Fig. 11, and the other end connects with the other binding post marked b, while one of the sliding contacts is connected to the binding post c, and the other sliding contact is connected with the binding post d.
When connecting in the tuning coil, only the post a or the post b is used as may be most convenient, but the other end of the wire which is connected to a post is left free; just bear this point in mind when you come to connect the tuning coil up with the other parts of your receiving set. The tuning coil is shown complete at B and it costs $3.00 or $4.00. A triple slide tuning coil constructed like the double slide tuner just described, only with more turns of wire on it, makes it possible to receive wave lengths up to 1,500 meters. It costs about $6.00.
The Loose Coupled Tuning Coil.--With a loose coupler, as this kind of a tuning coil is called for short, very selective tuning is possible, which means that you can tune in a station very sharply, and it will receive any wave lengths according to size of coils. The primary coil is wound on a fixed cylinder and its inductance is varied by means of a sliding contact like the double slide tuning coil described above. The secondary coil is wound on a cylinder that slides in and out of the primary coil. The inductance of this coil is varied by means of a switch that makes contact with the fixed points, each of which is connected with every twentieth turn of wire as shown in the diagram A in Fig. 12. The loose coupler, which is shown complete at B, costs in the neighborhood of $8.00 or $10.00.