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. 



848 



