Radiometric Measurements of Constants of Indicators. 47 



materially in this respect. Between the leveling stand of the radio- 

 micrometer and the base on which it rested, various insulating mate- 

 rials were introduced at six points to absorb vibrations. 



The completed radiomicrometer, candle and scale being at a meter's 

 distance, gave a deflection of 20 cm., the half -period being nearly 10 

 seconds. In making this determination of the sensibility of the instru- 

 ment the radiation was passed through a glass window 1 mm. thick, 

 and the tube containing the suspension was not evacuated. Evacuat- 

 ing the tube and passing the radiation through a rock-salt window 

 would increase the sensibility about six times, and the period would 

 become somewhat shorter. 



Since the receiving vane has an area of 4 sq. mm., the sensibility per 

 square millimeter of exposed vane is 5, and the full period 20 seconds. 

 When making this determination, owing to the difficulty of properly 

 shielding the junction not in use but serving as compensator, it is 

 thought that the value of 5 for the sensibility is somewhat low. Prac- 

 tically every junction that was tested showed that the sensibility was 

 approximately one-fourth of the whole period in seconds. The sensi- 

 bility value as given by the candle has but little meaning, owing to the 

 varying intensity of the light emitted. The light from the Nernst 

 glower, burning at 0.8 ampere and 120 volts, dispersed into the visible 

 first-order spectrum 6 inches long at slit Si of the spectroscope, gave at 

 X = 0.95/z a deflection of 250 mm. when focused on the junction of the 

 radiomicrometer. Both slits are 1 mm. in width, and the scale 12 feet 

 from the mirror. Considering the high resolving power of the grating, 

 it will be seen that our radiomicrometer is very satisfactory, both as 

 regards sensibility and period. It enabled us to make quantitative 

 measurements of radiant energy from X = 0.4/z to X = 2.0^. 



For the same source of light energy, the radiomicrometer always gave 

 the same deflection and returned quickly to zero, showing that our 

 efforts to eliminate magnetic materials from the wire loop had been 

 very successful. The conduct of the completed instrument has demon- 

 strated that the suspended system is very free from paramagnetic 

 disturbances. The diamagnetic materials composing the junction were 

 all arranged at right angles to the lines of force of the magnetic field, 

 and in this way diamagnetic disturbances were reduced to a minimum. 

 The use of the radiomicrometer has shown that the heat capacity of 

 the junction is sufficiently low to be within the desired limit. The 

 compensating junction and its insulation from temperature changes 

 practically removed any drift of the zero-point. The zero-point for 

 weeks at a time would not drift more than 10 cm. on either side of the 

 zero. In case there was any drift during the measurements, it was 

 always very slight and the proper correction could easily be applied. 

 The insulation from vibrations made the reflected beam upon the scale 

 fairly steady, even when there were rather violent disturbances in the 



