Radio age (Jan 1927-Jan 1928)

12 RADIO AGE for July-August, 1927 The Spectrum of Radiation By ELMORE B. LYFORD A LARGE part of our daily life depends upon vibration, in ■ one medium or another. Sound, and our powers of hearing, depend upon vibration of the air around us. Light, and our powers of vision, depend upon vibrations in the ether. The radio we listen to in the evening is actuated by waves which are transmitted by this same ether. Radiant heat, X-rays, and the electric current in most of our homes — all depend upon vibrations, or waves, ofone frequency or another. What the average person does not realize, perhaps, is that all of these waves, or vibrations, are identical in their form, and differ from each other only in frequency, or wavelength. The relation of these different vibrations, one to another, and the order in which they fall when arranged according to frequency, may therefore be interesting. When comparing vibrations of greatly differing frequencies, the most convenient measure is the octave, that is, the frequency scale of accoustics. An increase of one octave means a doubling of the frequency, regardless of the absolute frequency, or starting point. To set up such a scale, let us choose as our starting point 16 cycles, or vibrations, a second, this being about the lowest frequency that can be recognized as a sound by the human ear. One octave above this would be 32 cycles a second, two above would be 64, and so forth. The third octave above our starting point is 128 cycles per second— middle C on the piano. Sixtyfive octaves will be necessary to include all the vibrations known to man. *&. itttrt , rjj , rtfT, /. ] A graphical representation of these sixty-five octaves is shown, and the position of various types of vibrations on this "scale" are marked. A little study of this chart will bring out several interesting facts. One of the most striking is that the range of visible light is less than one octave out of the entire sixty-five. The ear is much more versatile than the eye, for it can hear vibrations over a band of frequencies nine octaves wide. At the bottom of our frequency scale — the slowest vibrations with which man deals — come the 25, 60 and 133 cycle alternating electric currents which are almost universally used for power and lighting purposes. The frequency band to which the ear is sensitive also starts around 16 cycles per second, and extends nearly nine octaves, up to frequencies in the neighborhood of 8000 cycles per second. These vibrations which affect our ear are carried by air as a medium, but all other vibrations are transmitted by waves in that intangible stuff which permeates all space, and which scientists call ether. Next higher in our frequency scale come vibrations caused by electric waves. These are caused by lightning flashes, certain forms of arcs, and "surges" such as are sometimes encountered on electric transmission lines. Just above and overlapping this band come the ether vibrations known to us as radio waves. Varying from 15,000 to 300 million cycles a second — from one to 20,000 meters, in wavelength — they cover a frequency band of fourteen octaves. This is the largest band of frequencies covered by any one class of vibration, and it is being gradually extended even more, particularly on the high frequency end. On higher frequencies than the waves classed as radio, but yet closely allied, are the Herzian vibrations. These vibrations were produced and measured before those at "radio" frequencies, and in a certain sense the work of Herz was the forerunner of the great development of the radio frequencies which has occurred in the last forty years. Between the Herzian waves and the very longest infra-red rays, next above, there is a gap in our scale which covers a range of about nine octaves. Vibrations in this range of frequencies, from about a thousand million to a million a second, have not yet been discovered, though there is no reason to suppose that they may not exist. Modern science knows no way of producing frequencies of this order, nor no way of detecting their presence if they could be produced. The characteristics which vibrations in this band of frequencies might have cannot even be deduced. If apparatus is ever devised which will produce and detect them, they may be found to be very valuable for the transmission of signals, or voice, or power — or they may be worthless for any practical use. The infra-red rays or vibrations above this unexplored region cover a band of about eight octaves. They blend then into the frequencies which constitute visible light, and which, as has been said before, cover a band less than an octave wide. "Light" waves of frequencies too high to be detected by the eye are called ultraviolet, and these have been detected by other means over a band of frequencies about two octaves wide. Vibrations in the infra-red range are sometimes called radiant heat, and are detected in the radiation from certain stars, and from the moon. Ultra-violet radiations are given off by an arc light, the brighter stars, and by other very hot bodies, including the sun. These ultra-violet rays are of importance in medicine as germ-killers, but in excess they are also more or less destructive to human life. Above the range of frequencies covered by the ultra-violet radiations there is another band which is so far unexplored and unknown, and above this comes the range of frequencies attributed to X-rays. These cover a very narrow band at about the 57th octave. These X-rays are very important in medicine, because of their property of penetrating otherwise opaque bodies, allowing us to take photographs of things inside. At the extreme end of our frequency scale we find the frequencies of the different radiations given off by radio-active substances such as (Continued on page 16)