Page:Popular Science Monthly Volume 88.djvu/852

 Measuring the Light of the Stars

By Joel Stebbins

Professor of Astronomy in the University of Illinois

Prof. Stebbins' remarkable measurements of the heat of stars have attracted the attentio7i of astronomers all over the world. Apart from the value of the results obtained, his work is interesting because it shows that astronomers are making use of modern technical advances, as in the case which he describes, sometimes before they are perfected for commercial purposes. — ■Editor.

��ONE of the standard problems of astronomy is the exact determina- tion of the amount of light that comes from each of the stars. Not that the knowledge of the fraction of a candle power of each star is of any interest or importance, but that the measures are valuable for future reference, especially to determine the gradual changes in light caused by the dying out or the brightening of these distant objects. Our own sun being one of a class of stars, the best clue to the life history of the sun may be given by a study of other bodies of the same kind. We also find in the sky numerous extraordinary objects, called short-period variable stars, which change in brightness by fifty per cent or more in the course of a few days, or even hours. Wanted: A Standard Eye For general purposes the unaided human eye is one of the best instruments fpr measuring the light of stars, and most f.orms of photometer depend ultimately upon the eye for a comparison of two lights. Because of the difference between individuals, however, there is no such thing as a "standard eye," and astrono- mers have long been waiting for some purely mechanical device which will register light intensities. Let us note that such an instrument is even more in demand for commercial work, especially for testing electric lights. At present the ordinary householder has to take the word of somebody else for the amount of light he is getting from electric lamps. The lighting companies accommodate us with meters telling how much current we use, but we have no exact measure of how much light they are delivering. City authorities contract for a number of lamps of say one thousand candle power each, but who knows after the lamps are installed whether they furnish a thousand

��or only eight hundred candle power?

We see that there is a real demand for an instrument which, held at a given distance from any lamp, will indicate just how much light is being emitted. Needless to say, many experimenters have attempted to perfect such an instru- ment, but so far without success. The underlying principle of these devices has been to make use of some substance which changes its properties under the influence of light. One of the most im- portant is the element selenium, a sub- stance in the same chemical group as sulphur. For more than a generation it has been known that the crystalline form of selenium changes its electrical resist- ance when exposed to light. Other sub- stances exhibit this same property, but none to such a marked degree as selen- ium. The ordinary arrangement is called a cell or bridge. Two wires are wrapped about an insulator, and on one face the selenium is deposited and then sensi- tized. The best method of sensitizing is a trade secret, but one standard method is to melt the selenium at four hundred and twenty degrees Fahrenheit, and then let it cool gradually, when it will crystallize and be light-sensitive. There must be a certain amount of mystery in the process, even to the makers them- selves, for none of them can furnish cells of a standard resistance, nor even two cells which are precisely alike. On the opposite page is shown an unmounted cell of the usual form. In the dark it has an electrical resistance of about five hundred thousand ohms, but on exposure to strong daylight the resistance drops to about ten thousand ohms, or only one-fiftieth of the original.

The principle of a selenium photom- eter is, then, to connect a selenium cell with a small battery and to measure the

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