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Page:Popular Science Monthly Volume 89.djvu/862

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A Quick Method of Measuring Light

The greater the size the greater the accuracy as a measur- ing instrument where this kind of a sphere is used

���A one hundred inch sphin liui in. ur u td lui h^lit- testing work. A sight box, large pho- tometer-bar, movable comparison lamp and scale graduated in millimeters are essential elements

��THIS qiiccr-looking ai)|>aratiis, sugges- tive of the rind of some Brobdigna- gian watermelon, is de\oted to the innocent purpose of measuring light. Wc say "rind" advisedly, for the sphere is hollow. It is of a dark color on the outside, and, like the melon rind, white inside, but there the similarity ends, for instead of pulp and seeds one finds at the center of the sphere a whitened fixture designed to hold the electric lamp which is to be tested. The instrument is located at the Nela Park lal)orat<)r\-, in Cleveland.

Light from the test lamji issues through an opal window in the outer shell and falls on a comparison-screen. In the illustration this screen is being obser\ed by the young lady sitting at the left. Inside the long box at her right is a sliding lam]) of known candlepower, against which the test l.unp in the s|>here is measured.

The a|>|)aratus is known as an I'lbricht Sphere; the original form was invented in Dresden by I'rof. K. rihrii hi. It operates

��on the well-known principle that the inte- rior of a whitened enclosure of this kind is of appro.ximately uniform brightness, and can be used to measure the total light out- put of a lamp, as distinguished from its candlepower in any one direction.

The principal application is in the measurement of large gas-filled lamps, which, on account of the peculiar coiling of their lilanu-nts, vary widely in candk'ixnver tlistrilnition and can be rated ciuickK' and accurateh' onK' on such an instrument.

This i)articular I'lbricht Sphere, one hundred inches in diameter, cost se\'eral lunulred dollars and is the largest in the country. The greater the size, the greater the accuracy as a measuring instrument, since the percentage of reflected light to absorbed light is increased.

The value of the I Ibricht invention as a time-saver ma>- be illuslr.ited b\- the fact that with its aid live mimites sullice to make measurements that would require fiilK' half an hour un<ier former methods.


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