ULTRA-SOUNDS

Greymouth Evening Star, 28 November 1927, Page 8

ULTRA-SOUNDS

REMARKABLE EXPERIMENTS.

One of the most exciting papers on experimental physics'that has appeared for some time is published in a number of the “Philosophical Magazine." The authors are Professor R. W. Wood and Mr A. T. Loomis, the former being the eminent American physicist and one of the few foreign members of the Royal Society. The experimenters have discovered that ul-tra-sounds have remarkable and unexpected properties. An ultra-sound bears the same relation to an ordinary sound that ultra-violet light bears on ordinary violet light. We are becoming familiar with the remarkable properties of ultra-violet rays; it seems now that we arc to expect equally remarkable properties in ultra-sounds. Just as ultra-violet rays have a shorter wave-length than the violet rays we see, ultra-sounds have a shorter wave-length than the sounds we hear. An ordinary sound can be made by causing a plate or diaphragm (as in a telephone) to vibrate, the vibrations being communicated to the surrounding air, which transmits them as waves of sound. If the vibrations are at the rate of 40,000 per second, they will cause audible sounds, but if the rate of vibration is over 40,000 (some place the average upper limit as low as 25,000) per second, the human ear will be unable to hear the resulting ultra-sounds. To produce an ultra-sound a plate has to be caused to vibrate with a frequency of over 40,000 per second. This can be achieved very neatly by exploiting an interesting property of certain substances (in particular, quartz) consisting of odd-shaped crystals. When these crystals are compressed electric charges develop on different parts, probably due to the uneven stresses set up in the oddshaped crystal structure; and, conversely, when suitably electrically charged they shrink or expand. If the opposite sides of a plate of quartz are given positive and negative! electric charges the plate will be distorted. If the charges are suddenly reversed to negative and positive, the plate will be distorted in the opposite direction. If the reversion is repeated rapidly, the plate is made to vibrate. The rate of vibration depends directly on the rapidity of the alternations in the electric charges. Now alternating electric charges can be produced by suitable electrical machinery with a frequency as high as 700,000 per second. If the quartz plate is connected to such a machine, it can be thrown into an intense state of vibration of frequency up to 500,000 per second; at higher frequencies the quartz is liable to be shaken into fragments. These vibrations cause “sound” waves in solids, liquids, and gases in contact with the plate, but thd' resulting “sounds" are utterly beyond the range of the human ear.. Ultra-sound waves were used for the detection of submarines during the war. Beams of waves of frequency about 40,000 were directed through the water towards submarines, the hulls of which reflected the beam as an echo. The times that the echo took to reach certain places enabled the position of the submarine to be calculated. The higher the frequency of the waves the more accurate is the determination, since higher-frequency waves do not spread as much as low-er-frequency waves, but unfortunately waves of frequency over 40,000 are absorbed by water, and beams of them do not penetrate very far. The wartime researches on ultra-sound waves tended to stop at frequencies of about 40,000 since waves of higher frequencies were not navally serviceable. Professor Wood uses a quartz plate submerged in an oil bath, and the waves are transmitted through the oil. Flasks, pipes, and so forth are submitted to the waves by dipping them in the oil. In many of the experiments the plate is submitted to .300,000, alternating charges per second at a pressure of 50,000 volts. The high voltage increases the intensity of the; vibration. The ultra-sound waves from the plate were found to exert a pressure of no less than five ounces on a glass disc about three inches in diameter when the disc was placed in a suitable position. One may compare this enormous wave pressure with the minute pressure exerted on the eardrum by the wave from a thunderclap or a musical instrument. The ultra-sound waves can be transmitted through glass threads a yard or more in length and less than onehundredth o,f an inch in diameter. If the end of the thread is held between finger and thumb, the vibration causes a burn, due to the minute but terrifically rapid poundings of the glass on the skin. In one case bright red spots similar to blood-blisters developed, which did not disappear fdr weeks suggesting that blood-vessels deep in the skin had been disrupted. A pine chip is burned and bored and caused to emit sparks, as if it were in contact with a red-hot wire. If a glass plate is held against the thread, a hole is bored through it, and minute particles of glass powder and globules of fused glass are flung aside. These experiments suggest that the ultrasound waves may be used for the production of concentrated but exceedingly intense vibrations, possibly for surgical purposes. It was found that living organisms having cells were killed owing to disruption, the waves setting up contrary forces in any body which is substantially larger than the wave-length. On the other hand, bacteria were: unaffected, their small size enabling them to vibrate as a whole and thus escape distorting forces. Fish exposed to the waves are quickly killed, a fact discovered during the war with the submarine detector waves. Mice are much less sensitive. Their death appears to be due to a rise, in internal temperature caused by the absorption of the waves in the body-material. The preliminary account of Professor Wood’s and Mr Loomis’s researches arouses the keenest anticipations, especially because the properties of ul-tra-sound waves suggest that they may usefully be applied to industrial and surgical purposes.