LIGHTNING ANALYSED

Taranaki Daily News, 6 October 1934, Page 17 (Supplement)

LIGHTNING ANALYSED

ANATOMY OF THE FLASH. “SLOW-MOTION” PICTURES TAKEN. The popular phrases “greased lightning’ and “blazing the trail’\ have been give an enhanced significance, through recent work in South Africa by Dr. B. F. J. Schonland and his. colleagues, writes Dr. A. S,^Russell in “The .Listener.” He has studied, the development of the lightning discharge by taking. and interpreting a large number of photographs of lightning flashes with a modification of the Boys lightning camera. This camera takes in a short tune a very large number of photographs which can be examined at leisure later. The principle is that of the slow-motion film, but the technique is so good that events which take place literally with lightning rapidity can be slowed down to appear as events of normal life. A thundercloud, it is known,, can act as an electrical machine which generates a surprisingly small amount of electricity before discharging it to the ground at a very high voltage. The quantity may be only 20 coulombs—say, the quantity which passes through an average electric lamp in about a minute—and this may be generated by the cloud in about five seconds. The voltage of the discharge, however, may be 500,000,000 volts, and it is this enormous potential difference which causes the dazzling and destructive effects of lightning.* The generator of power in the cloud is the wind, which blows up with great force vertically below it, like a gale roaring dp a chimney. The most usual type of lightning discharge seen in South Africa is also the most interesting. According to the photographic evidence the .downward stroke of the discharge—from the negativelycharged cloud to the surface of the earth —is different from the upward stroke from earth to cloud. The first of the downward-moving strokes—the leader, or.

I . r trail-blazer—is also quite different from the subsequent downward strokes. This first downward stroke travels from the cloud about 50 yards towards the earth, /? then its light fades away; the stroke . seems to pause for* about the tenthousandth of a second; then the streamer goes downwards over the old track / without showing much light but blazes forth as it enters fresh air. After about 50 yards of fresh air it pauses again for the ten-thousandth of a second. Then, as before, it resumes the blazing of a g further 50 yards. In this step-wise fash- . ion the main part of the discharge even- ‘ tually reaches th§ ground—“eventually” occupying about a hundredth of a second —while part spends itself in the numetous and well-known branches which are /// characteristic, of this kind of lightning// The instant the leader strikes the / g ground a brilliant flame sweeps upwards, towards the cloud, retracing the main path blazed intermittently by the leader from the cloud, and chasing and overtaking those branches of the leader which had failed to reach the ground. There is no pause in this upward sweep; it takes only a few hundred thousandths of a second and is in consequence in. marked contrast to, the downward discharge.

A cloud, it was found, may discharge ' itself by subsequent flashes to the first, but they also are in marked contrast to it. They appear in the. photographs as continuously-moving dart-like flashes. // J The intermittent flashing downwards /is always the very start of the. discharge,; never part of the later phenomena; the ' .{ ’5 dart-like continuous flashes are always // later, never at the start. Dr. Schonland has estimated the wave-lengths of ’the /jj?/ electro-magnetic radiation and the sound • waves which should be produced by this new, interpiittent, downward-moving leader of the discharge. They correspond. ; with a ripple in many atmospherics and •.£•-/ with the sound which is sometimes re- X ported as accompanying a dose dis- • / charge; that of tearing linen. These'accompaniments are thus more intimately explained than they have been previously.,. -