L. T. KNIGHT
The receiving set of a wireless station comprises the coherer, the relay, the decoherer, the sounder, the weak current and strong current batteries, coherer condenser and the tuning coil. A switch is arranged to connect the receiving set to the aerial wire when desired.
One form of coherer consists of an exhausted glass tube containing two silver plugs fitting snugly in the tube with well polished and slightly sloping ends. The space between the plugs varies from 2 to 4 millimeters in length and contains the filings of pulverized and oxidized silver and nickel alloy. The silver plugs are connected to metal tips or caps at the end of the coherer, which serve as connecting points. The coherer is not adjustable so far as the location of the plngs is concerned, but the sensitiveness is the greatest when the narrow part of the wedge is down. But this is not always the best working position for regular receiving.
The best relay is of the polarized type, with the coils of the magnet wound to a resistance of from 4,000 to 10,000 ohms. The relay must be well balanced, absolutely sure in action and quick to start. A first-class relay will operate with one volt difference of potential through a resistance 50,000 ohms.
The decoherer is a high frequency vibrating hammer constructed on the ordinary vibrating door-bell principle. The weak current battery used in the coherer operation is a dry cell of very low amperage and one to one and one-quarter volts. The coherer condenser bridges the relay windings and battery, is of very small capacity, and when properly placed serves to shut off any static charges that might flow in from the aerial wire to and through the coherer and relay and prevent operation.
The sounder is the ordinary Morse device, operated by several cells of dry battery. The tuning coil is a variable contact device patterned after the one described in the previous chapter.
The adjustment of receiving instruments requires care and patience, particularly in the relation of the relay and coherer, and the sounders and the decoherer. The sensitivity of the coherer varies with the degree of oxidization of the silver-nickel fillings, as well as the size and distance between the plugs. The normal resistance of the coherer is very high and the amount of current from the little dry battery is not sufficient to pass through to the relay windings. But when the wave enters by way of the aerial wire this high resistance is broken down by the critical potential and becomes approximately about 3,000 to 5,000 ohms. In this state the current is allowed to pass into the relay circuit and operate the relay. This, in turn, throws in the sounder and the decoherer, the circuits of which are shown in accompanying sketch. When the decoherer operates the relay circuit is opened and the relay returns to its normal position, and of course at the time the decoherer operates the sounder also operates. The decoherer is more active in operation than the sounder, and this permits the relay and decoherer to cause the sounder to give dashes as well as dots.