124 VACUUM RADIOMICROMETER. 



is provided to bring the mirror in any desired position. The loop for 

 the radiometer-radiomicrometer is shown in fig. 92, a. The Bi is about 

 2 by 0.15 by o.i mm., soldered to a thin sheet of copper 8 by 0.6 by 

 o.i mm. In one case a sheet of mica of the same area was used and 

 extended below the copper sheet, which was then only 2 mm. long. 

 This loop also weighed less than 10 ing. As a radiometer the time 

 required for a maximum deflection was 25 seconds, and its sensitiveness 

 was about 3 to 4 cm. per square millimeter area of vane for a Geryk 

 pump vacuum, which was not high enough for maximum radiometer 

 sensitiveness. This same loop used as a radiomicrometer had a sensi- 

 tiveness of about 5 cm. deflection per square millimeter area of vane, 

 while its maximum deflection was reached in 8 seconds. The combina- 

 tion of these two was no better than the radiomicrometer, simply 

 because of the fact that the periods were different and the magnetic 

 moment of the radiomicrometer obliterated the radiometer effect. 

 Another loop twice as heavy had a half period (time of maximum 

 steady deflection) of 5 seconds when used either as a radiometer or 

 radiomicrometer. The actual sensitiveness was much lower than in the 

 preceding loops. In this case the radiometer contributed 1 only 15 per 

 cent to the total deflection, while the sensitiveness of the radiomicro- 

 meter was increased 50 per cent by placing it in a vacuum. 



In fig. 92, b, is shown the loop for the radiomicrometer. The dimen- 

 sions of the Bi-Sb junction are indicated above. Its half-period in air 

 was 20 to 25 seconds, and the deflections in centimeters per square milli- 

 meter for meter, candle, and scale were 3.6 cm. On exhausting the 

 instrument with a Geryk pump the half-period dropped to 12 to 14 sec- 

 onds, while the. deflection increased to 5.5 and 6 cm. per square milli- 

 meter of exposed area of vane, and as the pumping progressed the 

 sensitiveness continued to rise. The result as a whole indicates that the 

 weight of the moving coil can be reduced to at least one-third of those 

 previously described and that its sensitiveness can be increased by at 

 least 70 per cent by placing the loop in a vacuum. Since a window is 

 used to increase the steadiness, this is no drawback, while in comparison 

 with the type of radiometer having a heavy vane, as already described, 

 it can be given a shorter period. 



The purpose in describing this form of instrument is not so much 

 to show the sensitiveness of the present one as to indicate directions in 



1 The method of testing is as follows : The radiomicrometer effect is first found 

 in the air. It is then exhausted and the deflection again noticed. The magnet is 

 then removed and the deflection obtained is due to the radiometer effect. The 

 magnet must of course be placed so that the radiomicrometer deflection is in the 

 same direction as that of the radiometer. 



