THE RADIOMETER. 



161 



It will be shown presently that this is about five times the sensitiveness of 

 Snow's bolometer, for which i mm. deflection (scale at 3 m.) indicated a 

 temperature difference of 7.5Xio~ 6 . In other words, this radiometer 

 would detect t 00 q 000 degree rise in temperature. But the period of the 

 radiometer was 6 times that of the bolometer, which is its weakest point in 

 radiation work requiring a short period. 



Table V. Sensitiveness of Radiometers. 



A comparison can also be made (table V) between light vanes (0.5X9 

 mm.) and heavy ones (1.3 X 10.5 mm.) at the same pressure but having 

 different quartz-fiber suspensions. The results show that while the light 

 weight vanes are more sensitive than the heavy ones (see table V), there 

 seems to be no limit to the sensitiveness attainable in either case, pro- 

 vided one does not consider the period. The idea of not considering the 

 period of vibration with sensitiveness seems reasonable, for by sensitive- 

 ness is meant the minutest quantity of radiation one can detect, assuming 

 one is willing to wait for the deflection to reach a maximum. In table VI 

 are compiled the most notable radiometers used in radiation work. The 

 use of a candle as a standard of comparison is questionable, but since the 

 sensitiveness of the various instruments varies by a factor from 2 to 20, 

 it is sufficiently accurate for the present comparison. In this table it will 

 be noticed that for the same period the various radiometers vary in sensi- 



