UNIQUE CIRCUIT.

Otago Daily Times, Issue 21244, 27 January 1931, Page 16

UNIQUE CIRCUIT.

CARRIER TELEPHONY. ON HIGH VOLTAGE LINE. “ Are you in favour of compelling municipalities to supply water and gas through the same pipe?” was one of the questions put to a political candidate by a merry interlocutor. “ Don’t talk nonsense,” replied the candidate (who has been talking nothing else the whole evening!). Well, drinking water and illuminating gas may not mix satisfactorily in _ one conduit, but the Victorian Electricity Commision is quite proud of the fact that it has made its 1 high voltage lines do duty as telephone lines—and that feeble speech-bearing currents can be accommodated safely in the one copper medium. POWER BEARER AND VOICE CHANNEL. “In the last few weelss (writes the Argus of January 1) the State Electricity Commission has opened one of the most remarkably telephone circuits in Australia, and a circuit to which there are few similar in the world. 7 lt is a ‘trunk’ circuit linking the commission’s officers and terminal stations in Melbourne with the main generating 'stations at Yallourn; but instead of being provided over an ordinary pair of conductors it is established along the new main transmission line, which, in addition to acting as a ‘ voice channel,’ simultaneously conveys thousands of horse power of electrical energy, from Yallourn to the metropolis at a pressure of about 132,000 volts. “ When the first transmission line to Yallourn was erected the necessity for constant and reliable communication between the works and the commission’s headquarters was recognised, and a special line of the orthodox kind was erected beneath the transmission line at a cost of many thousands of pounds. In the last few years, however, the art of ‘ carrier wave ’ telephony has been so rapidly developed that when the extensions to the power house now being undertaken were designed it was found practicable to avoid the second telephone circuit which has become necessary to deal with increasing volume of departmental traffic by establishing a carrier wave telephone circuit along the transmission line itself. The cost of a second ‘ physical ’ telephone circuit was thus avoided, but in a degree of reliability of communication never attainable on an ordinary circuit was assured. CARRIER TELEPHONY NOT NEW, “ Carrier wave telephones have been in operation between the capital cities of Australian for some years, but the problems overcome in establishing the Yallourn circuit far transcend* those encountered in ordinary telephone practice, because the medium over which the ‘ carrier ’ operates is not an ordinary telephone line easily handled and carrying at,all times feeble currents, but a conductor of electricity in bulk, at a pressure sufficent to cause instant death to any person coming into contact with it. Carefully tested and elaborate equipment at each end of the line was therefore necessary to complete the telephone circuit; and at the same to avoid the disastrous results which would follow a leakage of the high voltage current from the line into the telephone instruments. The method by which this isolation has been achieved depends on the fact that the current used to carry the telephone conversation is an alternating one which changes its direction, no fewer than 100,000 times a second, while that which carries the power along the line alternates only 50 times a second. A condenser is connected to each end of the line and to the telephone instruments. The rapidly surging speech-bearing currents, feeble though they are, can pass across this condenser, which constitutes a break in the circuit, as if it did not exist; but the powerful high pressure currents carrying the energy on the line are completely stopped b£ it. RELIABILITY. “The condenser is similar in its fundamental principle to the fixed condensers in a wireless set, but radically different in design. It consists of a thick walled bottle of porcelain about five feet high, coated externally and internally with sheet metal. It provides a striking margin of safety. A pressure of 300,000 volts must be applied to it before it will ‘ flash over ’ —that is, before sparks will leap round the surface of the porcelain between internal and external conductors, and a very much higher pressure is required, to puncture the porcelain. Two of these condensers are used in series at each end of the line. This means that each is required to withstand only half the total pressure of the current, and the margin of safety is thereby doubled. “A striking feature of the equipment is the reliability of the circuit. It has worked satisfactorily with full load on the transmission line, and even the marked electrical surges developed _ in the line does not interrupt communication. Linesmen may break the line, and even dismantle the conductor between two of the towers without interrupting the circuit. Should a severe storm or any other factor cause a break in the line at any time this feature will be most valuable, because the certain communication assured would greatly expedite repair work.”