568 RADIANT EFFICIENCY OF ARC LAMPS [Cn. XIII 



falls on the comparison screen either directly, or after passing 

 through an 8 cm. layer of water, as the case may be. The other 

 patch of light is robbed of all of its infra-red by the system of 

 prisms and lenses shown in fig. 308. 



The energy in these two light patches was measured by a radio- 

 micrometer. The screen S 2 could be set to remove all of the infra- 

 red radiation to any desired point in the spectrum. In this work, 

 after careful experiment, it was decided to adopt the wave-length 

 .6&(i as best representing the dividing line between the visible part 

 of the spectrum and the infra-red. The screen was accordingly set 

 to remove all radiation of greater wave-length than this. 



By this method it was possible to measure the energy repre- 

 sented by the total radiation of the arc, and that of the visible 

 portions. It was also possible to insert a water-cell between the 

 source L and the radiomicrometer and compare the light energy, 

 with that part of the energy passing through an 8 cm. layer of 

 water. In order to simplify the discussion, the total radiation of 

 the arc is called R, the portion getting through the 8 cm. water-cell 

 is called W and the luminous energy is called L. The measure- 

 ments were made in such a way that the ratio of L/R or radiant 

 efficiency was determined, or else the ratio of L/W was measured. 



In addition to these values, the transmission of layers of water of 

 different thickness was measured, that is, the ratio of W/R was 

 determined. This ratio is called "the Water-Cell Efficiency" and 

 was determined for a layer of water 8 cm. thick. The transmission 

 of layers of other thicknesses is shown in 849. 



777. The results of these measurements for various sources 

 are shown in the table ( 778). The most important values are for 

 the positive crater of the right-angle carbon arc and for the right- 

 angle arc with alternating current. 



The positive crater shows a radiant efficiency (L/R) of roughly 

 10%, that is 10% of the energy radiated is visible as light, the other 

 90% of the energy is mostly in the infra-red. "The water-cell 

 efficiency" (W/R) varies from 18% to 28%, averaging roughly 

 25%, that is, one quarter of the energy radiated gets through the 



776a. See H. P. Gage, The Radiant Efficiency of Arc Lamps, Physical 

 Review, Vol. 33, p. in, Aug., 1911. 



