Page:Popular Science Monthly Volume 88.djvu/853

 Popular Science Moyithly

increase of current, due to light, by means of an ammeter or galvanometer. However, there are several difficulties in this simple process. When selenium is exposed to a strong light, some minutes or even hours are required for the re- sistance to return to its original value. Selenium is extra sensitive to red light, and so does not give directly a measure of how bright a light would appear to the eye. For instance, a carbon filament lamp with its yellowish light will affect a selenium cell just as much as a much whiter tungsten lamp of double the candle power. Finally and worst of all, selenium is very ir- regular in its action, and no experimenter has yet solved to his own satisfaction the mysteries of this element.

The Selenium Cell Is Packed in Ice

As applied to the stars, however, many of the ordinary difficulties disappear. Star light is so faint, even at the focus of a large telescope, that the slow recovery of the selenium is not such a drawback; next, since we are usually concerned only with variations of light, it matters little which color is used ; and lastly the irregu- lar action may be controlled some- what by keeping the selenium at a low uniform tem- perature. Strange as it may seem, when the light of stars is to be meas- ured, a selenium cell at the end of the telescope is surrounded with an ice pack, the ice being renewed every day in sum- In the circle is shown a selenium cell and ice pack attached to the

telescope The oval diagram above the circle pic- tures the system of Delta Orionis, show- ing orbit, eclipse positions, and compa- rative size of the sun

To the right — Un- mounted selenium cell, natural size

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