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 1890, and a coherer itself was first 

 used for laboratory experiments by Sir OUver Lodge in 

 this country in 1894. In the following year it was used 

 by Professor Popofi in Russia as a detector for aero- 

 electrical discharges, and in 1896 was in use by Signer 

 Marconi as a detector in practical wireless w'orking. 

 This filings-coherer, along with a few other detectors 

 whose working also depended on coherer-action, held 

 the field for some years before being supplanted by more 

 sensitive arrangements. 



They were first succeeded by Marconi's magnetic 

 detector, which was introduced in 1901, and quickly 

 followed, though not supplanted, by various electrolytic 

 devices, of which the first practical detector was intro- 



duced in 1903, in the United States, by Professor 

 Fessenden. 



The magnetic and electrolytic detectors could only 

 be used in conjunction with a telephone receiver — that 

 is, the message was not printed on a tape as dots and 

 dashes, which was the case with coherer devices, but 

 was read by ear as buzzing sounds in a telephone 

 receiver, the duration of the sounds signifying the dots 

 and the dashes of the Morse Code. 



At first this fact of not being able to have the message 

 printed on a tape was thought, by those not directly 

 acquainted with practical working, to be a great draw- 

 back, as it was said that there could be no certainty 

 that the operator wrote down the message exactly as 

 received ; whereas with the dots and dashes printed on 

 a tape there could be no doubt of the matter. The 

 writer well remembers many forcibly expressed opi- 

 nions from that point of view when it was first suggested 

 in the Navy that aural reception should supplant 

 recorded signals. However, the magnetic and electro- 

 lytic receivers were much more sensitive and reliable 

 than the filings-coherer, and it was soon found that this 

 advantage, as well as that of the operator being able 

 to read messages to a certain extent in spite of atmo- 

 spheric and other interference, far outweighed the dis- 

 advantage of not being able to check what was received 

 by reference to a printed tape. 



Tlfe telephone receiver consists of an electro-magnet 

 fixed close to a circular metal disc supported round the 

 circumference. A comparatively slowly alternating or 

 pulsating current flowing through the coils of the magnet 

 alters the magnetic field in such a way as to vibrate the 

 disc and produce sounds in the ear. The instrument 

 is one of the most sensitive current detectors, but cannot 

 respond to currents which oscillate at such very high 

 speeds as those produced by wireless waves, so that 

 some device must be introduced which will utiUse the 

 energy of these currents in such a way as to produce 

 currents suitable for operating the telephone. 



In the Magnetic Detector (see Fig. 2) the minute 

 osciUatory currents received were passed through the 

 coil AB, and so altered the magnetic field as to produce 

 currents in the coil TT, suitable for working the tele- 



