Statistics – Applications
Scientific paper
Jan 1931
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1931natur.127....3d&link_type=abstract
Nature, Volume 127, Issue 3192, pp. 3-4 (1931).
Statistics
Applications
Scientific paper
IT is a remarkable fact that in spite of the keen interest displayed in selenium ever since the discovery of its response to light fifty-eight years ago, no complete treatise on its properties has appeared in English until now. Students have had to rely on the small German handbooks of Ruhmer and of Ries, or the compendium, "II Selenio", published by Bianchi in 1919. If Mr. Barnard had done no more than compile his 50 pages of excellent bibliography, he would have conferred a consider able benefit on the scientific public. But his book does much more than that. The study of the properties of selenium has reached a peculiar position. The reaction following upon the earlier claims of inventors which failed to materialise has been so severe that the excellent work of more recent days has had to disguise itself, so to speak, under pseudonyms. Thus, television by means of infra-red rays and selenium is called 'noctovision', and a powerful London company for the manufacture and utilisation of selenium cells is known under the name of 'Radiovisor'. There has never been a time at which more selenium cells were constructed and used, or when such rapid and solid progress was made in the study of their pro perties . But few dare confess to it. It is the twilight of selenium, the eclipse of the 'moon-element'. When, in 1872, the light-sensitiveness of selenium was discovered, Mr. Shelford Bidwell said at the Society of Telegraph Engineers: "Mr. Preece has told us that by the aid of the microphone the tramp of a fly can be heard, resembling that of a horse walking over a wooden bridge; but I can tell you something which, to my mind, is still more wonder ful?that by the aid of the telephone, I have heard a ray of light fall on a bar of metal". Everyone would have expected that after such an introduction the combined efforts of the scientific world would have sufficed to unravel the secrets of the marvellous substance which presented such a phenomenon. Yet, fifty-five years after, Prof. Gudden, who has spent a lifetime in studying photo electric phenomena in crystals, says: "It is as hopeless to expect to understand what happens in selenium from the observation of selenium cells as it would be to determine the laws of vibration from the investigation of a creaking hinge". Prof. F. C. Brown is one of those who spent many years investigating large single crystals of selenium, but his work has not yet reached a satisfactory conclusion. Gudden and Pohl have, at all events, established one basic fact, namely, that in the in sulating red form of selenium the effect of light upon the conductivity is both instantaneous and linear, though reckoned in micro-micro-amperes. These red crystals have, according to Kyropoulos, a refrac tive index so high as 3.5 and obey Maxwell's law of refraction. Much of the work on selenium of the next fifty years will no doubt be done on red selenium. When the D.C. conductivity of a substance is to be studied, we must, unfortunately, have electrodes, and selenium with electrodes constitutes a 'selenium cell'. It should not surpass the ingenuity of our physicists to make the monoclinic crystals of con ducting selenium arrange themselves in a regular and reproducible order, and to find an electrode, say of carbon, which has no chemical effect upon the substance. We know that the square-root law of light-action is obeyed at faint illuminations down to very low values, and that is some foundation. There used to be a slogan in the Cavendish Laboratory at Cambridge when anything difficult had to be undertaken: "Do it with selenium!" (pronounced, in defiance of Greek etymology, "selennium"); and Mr. Barnard certainly enumer ates an imposing array of results achieved by the 'creaking hinge'. Fire and burglar alarms, automatic machines for counting stamp cancella tions and even carcases, street lighting and train control, the mechanical estimation of fog and smoke density, and the control of camera shutters, are some of the successful applications of selenium. Cox's magnifier of cable signals is fully described, as is Symonds's ingenious device for counting inter ference fringes. This and the optophone could not possibly function with any form of photoelectric cell. When we come to the applications of intermittent light, we are on less difficult ground, for all con siderations of the zero in the dark can be eliminated or disregarded. The only element that matters is the amplitude of the response. It has been found that selenium responds to notes of a frequency of 12,000 hertz. It is therefore capable, in spite of its 'lag', of following the whole gamut of the notes used in music. Theoretically, its response ought to vary inversely as the pitch, and a correction would have to be applied to make the higher notes stronger. This would be incon venient but for the fact that every reproducer devised so far must be corrected in one sense or the other. Usually it is the lower notes that want strengthening. The correction is the easier for the fact that there is plenty of current to spare. Prof. Thirring showed recently that selenium gives from 1000 to 1,000,000 times as much response as any photoelectric cell. A most valuable feature of this excellent book is the collection of chemical, optical, and electrical constants of selenium. Everyone will naturally turn to these tables for information as to the re sistivity, only to be told that it lies somewhere between 105 and 1010 ohms/cm.3 The figure 106 given by Siemens might have been quoted as an approximate practical guide.
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