SCIENCE SIFTINGS

New Zealand Tablet, 23 September 1920, Page 46

SCIENCE SIFTINGS

(By "Volt.")

Wireless Explained. Mile-long waves which, travel so fast that they could go round the world seven times in less than a secondthese are the things we are reading about daily —the electric disturbances set. up in space, or in the "aether" which fills space, by wireless aerials spotted all over the world (writes T. Thorne Baker in the Daily Mail). If you strike the middle Con a piano, and the C an octave below, a" person standing some distance away will hear both notes simultaneously, yet the middle C sets up twice as many air waves a second as the lower C; the sound travels at the same speed, the wave-length is different. Just the same with wireless signals. One transmitting apparatus may send out waves a thousand metres long, another five thousand metres. Both travel at the same rate, about 186,000 miles per second, but the longer waves are more suitable for long-distance transmission. A metre is rather more than a yard, and the early wave-lengths employed in wireless were 300 and 600 metres: the former for short distances, the latter for longer ones. Very much longer waves are in frequent use to-day. We often read of people who are "listening-in" for messages, and that they get into touch with—are able to listen tothe signals from some particular station by tuning. We all know how a certain glass, cup,' or vase in a room will ring in sympathy with a certain note struck on the piano because, if struck or tapped, it would itself vibrate and give out the same note. This phenomenon of resonance will cause anything to vibrate whose natural rate of vibration happens to be that of the note struck. A tuning-fork, for instance, which gave the note C would vibrate, very faintly no doubt, if held near a musical instrument on which the same C was struck. Now imagine a tuning-fork the prongs of which could be lengthened or shortened at will, so that whatever note was strucki.e., whatever length of musical wave was sent outyou could adjust the fork to respond to it. Translate sound-wave phenomena into electro-magnetic (or wireless) wave phenomena, and your tuning-fork becomes the tuning inductance with which you can make your receiver respond to whatever length of wireless wave the station is sending out. For good technical reasons wireless wave-lengths are measured in metres, and when we read of a thou-sand-metre wave it means that the length of each disturbance sent through space with the inconceivable velocity of 186,000 miles a second is some thousand yards m length. Seventeen of such waves would bridge the space between two wireless stations 10 miles apart The receiver would tune his circuit to respond to a thousand metre wave-length. These waves do not follow each other with the grandiose solemnity of sea waves; in wireless telephony a hundred thousand of them may come flashing by in the course of a second ho delicate is the receiving apparatus of to-day, that little interference is caused through the hundreds of messages that are crossing space at one and the same time with many a different wave-length Methods of tuning are highly refined though intensely simple. Directional devices have also helped considerably m this way. Yet it seems wonderful that while a separate pair of wires is needed for ever v telephone circuit at work, innumerable people could speak by wireless through the common aether without interference, save that "listeners-in" would be abe to gather up innumerable fragments of their conversation