THE WORLD OF SCIENCE

Te Awamutu Courier, Volume 53, Issue 3827, 30 October 1936, Page 11

THE WORLD OF SCIENCE

WIRELESS WAVE GUIDES. UNDERGROUND CABLES. The provision of supplies of energy and the means of transmitting this energy from place to place are two matters of great importance in the present age. So far the only way in which we transfer appreciable quantities of energy over long distances is either by transporting unburned fuel (as in the case of coal or oil), or by transmitting the energy by highvoltage electric currents along wires ■ supported on special insulators. * There is a wide interest in this question of the transmission of energy, and every new posibility is eagerly discussed. A few years ago we were told that to mark a special occasion a hall 'in Sydney was to be lighted by means of energy transmited by “wireless” from a yacht in the Adriatic. After the event, the less sceptically minded were a little disappointed to realise that this had meant nothing more than that a wireless signal was to be used to work a relay and operate a light switch. The era of wireless transmission of power had not yet dawned. The conveyance of energy by means of electro-magnetic waves is, of course, not novel, since all the energy received from the sun comes in this way. But the earth receives less than one thousand millionth part of Jhe energy radiated from the sun; so that if we regard the sun as a power station designed to supply energy to its own planets it must be admitted that the method of distribution is somewhat wasteful. For similar reasons the possibility of receiving our supplies of energy as well as our news from broadcasting stations is not likely to be realised in the near future. It is a large station that radiates energy at a rate of more than a few horse power, and this energy spreads out in all directions, so that the amount collected by any one receiving set or station is indeed infinitesimal. Electro-magnetic waves are of all wave lengths, from those used in wireless, which may be a mile in length, down to cosmic rays, less than one-mjllionth of an inch in wavelength. A beam of short waves does not spread out or disperse so much as the longer ones, so that by using short waves, as in “beam wireless,” it is possible to project the radiated energy chiefly in one direction. But, according to modern physical theory, the tendency to disperse, or to become spread out as they go, is a fundamental property not only of beams of radiated energy, but even of streams of particles. At present then there seems no probability that a projected beam of waves will take the place of a current along a wire as a means of conveying energy. Nevertheless, the importance of electromagnetic waves, as a means of communication, in broadcasting, carrierwave telephony; &c., lends an added interest to every new method of transmitting these from place to place. The latest development in this field is due to work carried out independently by Dr Southworth, of the Bell Telephone Company, and Dr Barrow, of the Massachusetts Institute,of Technology. The work, still in an early, experimental stage, may be described as the application of the principle of the speaking-tube to elec-tro-magnetic waves. In the speaking tube the sound waves are prevented from spreading by reflection at the inside of the tube, and thus confined, the sound is transmitted along the tube. Similarly, the very short “wireless” waves are confined within a pipe along which they are transmitted. These pipes are referred to as wave-guides or electro-mag-netic pipe lines. PRINCIPLE OF WAVE'GUIDES. It is well known that metals reflect light waves, and broadly speaking, it may be said that substances such as metals, which will convey electric currents, will not transmit electro-mag-netic waves, but will reflect them. Very short waves, such as X-rays, will penetrate metals, but for waves longer than ordinary light waves the reflection from a smooth metal surface is almost complete. The waveguide is simply a pipe of metal, and ■ the short electro-magnetic waves are prevented from escape by reflection at the walls and are transmitted it like the voice in a speaking tube. The end of the tube may be flared out into the form of a horn, and so project a beam of these waves in any desired direction. The diameter of the pipe must be somewhere in the same range of size as the length of the waves, so that for pipes of practicable size the waves must be only a few inches long, and have a frequency of not less than 2000 million vibrations per second, his range is outside that at present * ed in wireless, but it ir'suggested that these very short waves will be useful, and in fact necessary, in the development of television. By filling the pipe with some insulating material, such as paraffin wax, a narrower pipe can be used to transmit waves of a given wave V

length, and, in this case, we have the signals transmitted along an insulated core surrounded by a conducting metal sheath, whereas, at present, of course, the signals are conveyed by a conducting wire carefully surrounded by an insulating cover. It is just in this novelty that some of the advantages of electro - magnetic pipe lines may lie. For it is clear that there is no need to provide insulating supports for the pipe, since it is the outer metal that prevents the waves from escaping. Moreover, just as it prevents the conned waves from escaping, so the metal will prevent elec-tro-magnetic waves from outside from penetrating, and causing interference. In the matter of efficiency, the experimenters say that signals are conveyed along these wave guides with no greater loss than that which occurs when signals are conveyed along telephone wires at present. As has been said, this work i s still in an early stage, and little can be said as to its future use. It does not seem likely to afford a means of transmitting appreciable amounts of energy, but in crowded cities with many sources of electrical interference, wave guides may afford a means of transmitting short-wave programmes from the studio to the aerial. For extremely short wave lengths the mouth of the wave guide itself may form the directional aerial from which the programme is transmitted. UNDERGROUND CABLES FOR HIGH PRESSURES. In a former article, which def scribed the oiled-paper insulated, lead sheathed electric cable, it was stated that these proved very unreliable when first used at pressure of 33,000 volts. This was very perplexing, as laboratory tests on the insulation used showed an ample margin of safety; and much research was at once devoted to the matter.

The cause of the trouble was at length shown to be the presence in the cable of minute pockets of air or gas occurring fortuitously between the layers of oiled paper. Air, although a reasonably good insulating medium, is inferior to oil in dielectric strength, and where insulation consists of alternate layers of oil and air there may occur a concentration of stress on the air which results in its ionisation, with consequent slow deterioration of the paper in its immediate neighbourhood. A defect of this kind, quite insignificant and undetectable at first, will gradually extend until after some months the cable breaks down completely at this point.

Very great care in excluding all trace of air during manufacture, as by impregnating the paper before winding and even by carrying out the winding process in a bath of oil, markedly improved the quality of the cables; but breakdowns still occurred in cables which were fully loaded. That is to say, as long as the cables were used to convey not more than about half the current for which they were designed they proved very reliable, but they began to give trouble soon after they were first used at their rated current capacity. In other words, a cable to carry 150 amperes had to have about twice the section of copper that was sufficient at lower voltages; and as this meant also an increase in volume of insulation and lead sheathing the cost of the cables become excessive.

An explanation of this peculiarity was found in the expansion that materials undergo when heated. Passage of current through the copper causes an increase in temperature and a slight expansion of both copper and insulation; and the lead sheath must swell a trifle to permit of this; and as lead is very lacking in elasticity it tends to retain its slightly enlarged size when the copper cools and contracts, thereby causing “voids” in the insulation which form the seat of the trouble.

Underground cables for threephase supply are usually of the three- ■ core type, three separate cores of stranded copper being separately insulated, wormed together, with the interstices filled with jute fibre, and then again insulated before being sheathed with lead. Hochstadter markedly improved the quality of cables by wrapping fine copper wires or gauze round each separate insulated core before they were wormed together. This actually increases the electrical stress upon the insulation around the conductor, but has a threefold advantage which proved extremely valuable. Firstly, this method entirely removes all electrical stress from the jute packing where voids are most likely to occur; secondly, the elasticity of the copper wrapping helps to prevent the occurrence of voids in the paper; and, thirdly, the construction ensures that all electrical stress upon the paper is truly radial, and everywhere perpendicular to the layers of paper, whereas without this construction there is at places a tangential component of electrical stress which appears to make minute bubbles of air imprisoned between the papers drift sideways and collect into pockets. Practically all 33,000 volt cables are now of the “H” type, as this is called; and this construction, coupled with care in manufacture, has resulted in cables of this pressure being now practically as reliable as those for lower voltages, and “H”

type cables are being used up to 66,000 volts pressure. OIL-FILLED CABLES. The Hochstadter construction,, however, is a palliative rather than a cure for the danger arising from voids, and with pressures of 132,000 volts, for which engineers are now wanting cables, something better was needed, and an entirely new type was brought out. v The oily compound used for impregnating cables is so viscous at ordinary temperatures as to be practically a solid. This is necessary, as otherwise wherever a cable was laid under sloping ground, the oil would gradually flow to the lower end, with danger of bursting the lead sheath there by hydrostatic pressure. But it is this very viscosity which makes the voids caused by the alternative heating and cooling of the cable so dangerous, since a limpid oil would promptly fill the voids. The new construction deliberately adopts a limpid oil, and moreover provides ducts within the cable, to ensure the ready flow of oil. Wfith single core cables the duct is formed in the centre of the core, the copper strand being wound with long lay round a central wire coiled into the form of a spiral spring. With three core cables, there are three ducts provided in the spaces between the cores. To prevent the oil from draining away to the lowest point of the cable this is divided by step joints into lengths of a few hundred yards, each length being connected by rubber tubing to one or more hermetically sealed concertina-like vessels filled completely with oil under a slight but definite pressure. As the cable heats up under load, the oil expands and flows into these vessels, which lengthen to accommodate it. As the cable cools, the oil flows back promptly and keeps the lead sheathing quite full. During manufacture, the cable is put into a vacuum chamber and every trace of air is sucked out before the oil is run in: and each factory length of cable as it goes out on its drum has a small oil vessel suitable attached to it. Thus there is no air in the cable when it leaves the factory, and no voids can be created during its use, so that danger from air pockets is entirely eliminated. Oil filled cables of this type have been now in use for about ten years at 132,000 volts pressure with excellent results; and 18 miles of such cable for a pressure of 220,000 volts have recently been brought into use in Paris. The only serious drawback of the oil-filled cable is its great cost. It is, therefore, not surprising that of late years much research has been devoted to other means of obtaining reliabi- ( lity. GAS PRESSURE. It hag long been known that the electrical strength of air and other gases is much increased when they are under pressure; and by raising the pressure within a cable to about ten atmospheres, any voids therein should become quite innocuous. Much work in this direction has been carried out of late years, and one type based on this principle is now on the market. There are technical difficulties in the way of applying pressure inside a cable, chief of which is the weakness of the lead sheath; so the method adopted is in essence to lay down a length of high-pressure steel piping of suitable size, and thread the cable through it, just as on a small scale house wiring is threaded through conduit. Then the steel pipe is pumped full of gas, usually nitrogen, at the requisite pressure, and the lead sheath of the cable promptly collapses as far as the insulation inside will allow, or, in other words, until the pressure set up inside is practically the same as that outside. The method appears very promising. The gas pressure required is only about that of a public water supply, so that no new problems are set up in the maintenance of the pressure pipe line; and breakdown from voids is a matter of such low development that an occasional fracture of the pipe line and consequent loss of pressure, should do no harm to the cable inside, if repaired as goon as convenient. There will be a little complication at the ends, where the cable leaves the pipe to connect to a switch; and special pipe fittings will be required every two hundred yards or so, where the factory lengths of cable are joined end to end. Apart from these, there appears to be nothing jn the scheme outside ordinary engineering experience. Though only recently brought into commercial use, the makers have had the system under test for some years now, and are now prepared to instal it under guarantees.

However, it still remains the case that underground cables for extra high voltages are very much more expensive than are overhead lines. In general, at 60,000 volts or over it is cheaper to take an overhead line round a city than an underground line through it; and it is only in the largest cities of the world that the use of these extra high voltage under ground cables can at present be justified.—E.V.C.