. Discovery. Science. DISCOVERY ' 207 working—that is, for messages being transmitted to a distant station at the same time as messages are being received from that or another station. In some of the latest designs the most sensitive possible receiving instruments are used, and the receiving aerial is quite a small affair in the form of a loop or frame, as by such arrangements it is easier to eliminate interference from the sending station and facilitate duplex working, at the same time avoiding the great expense of a number of high masts. In the case of Carnarvon, the receiving station is at
. Discovery. Science. DISCOVERY ' 207 working—that is, for messages being transmitted to a distant station at the same time as messages are being received from that or another station. In some of the latest designs the most sensitive possible receiving instruments are used, and the receiving aerial is quite a small affair in the form of a loop or frame, as by such arrangements it is easier to eliminate interference from the sending station and facilitate duplex working, at the same time avoiding the great expense of a number of high masts. In the case of Carnarvon, the receiving station is at Towyn, some thirty miles away. The aerial at Carnarvon consists of twenty wires, each Cohere* _LL 3- 3,800 feet long, supported by ten masts 400 feet high. The actual operating is carried out at Towyn, the signalling key being connected electrically by wires to the transmitting apparatus at Carvarvon. In small stations, such as those in ships, the same aerial is used for transmission and reception, being switched over to the sending or receiving apparatus as required. When signals are received from a distant station, the electric currents produced in a receiving aerial by the waves radiated from the distant station are extremely small, and apparatus must be used to enable these minute currents to release sufficient energy for working some sort of recording arrangement, whether it be one for recording sounds in a telephone receiver, or marks on a paper tape. The apparatus first used was the coherer, which consisted of a sealed glass tube in which were two metal plugs separated by a small space containing metallic filings, as shown in Fig. i. Oscillatory currents have the property of very greatly reducing the electrical resistance of the little heap of filings, which are then said to have cohered together, and in this state can allow a current from a cell to flow through them, and work the recording apparatus. This coherer action was enunciated by Professor Branly in France in
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