VACUUM RADIOMICROMETER. 



which further improvements are possible. On account of the difficulties 

 in preparing a non-magnetic loop the final word has not been said on 

 the subject of combining the radiometer and the radiomicrometer. At 

 the present writing the best combination was only about 20 per cent 

 more sensitive than the use of the instrument as a radiomicrometer 

 only. 



For detecting electric waves the system can be made still lighter, 

 while the heating arrangement can be brought close to the junction. 



In conclusion, the fact may be emphasized that in the present test of 

 sensitiveness only that radiation which passed through a slit and fell 

 directly upon the vane was measured. If this precaution had not been 

 taken, the results would have been much higher and would have been as 

 misleading as those obtained by several other investigators. 



The sensitiveness of the bolometer varies as the square root of the 

 exposed surface. The writer has found experimentally that the sensi- 

 tiveness of a radiometer is proportional to the area of the exposed vane, 

 and hence is justified in expressing his results for unit (sq. mm.) area. 

 The use of a candle as a standard is questionable, but since the sensitive- 

 ness of the different vanes differs by a factor of from 2 to 25 the candle 

 is a convenient source for comparison. 



