30 ABSORPTION SPECTRA OF AQUEOUS SOLUTIONS OF CERTAIN SALTS 



This entire system was suspended in the interior of a glass tube, the tube 

 being closed by a ground-glass stopper, and provided with suitable windows 

 for exposing the junction and observing the mirror. The upright tube was 

 provided with a side tube for evacuation, and by a method devised by Pro- 

 fessor Pfund a very high vacuum could be obtained and maintained for any 

 desired length of time. By suitably turning the ground-glass stopper in the 

 top of the glass tube, the loop of copper wire, mirror, and, indeed, the whole 

 system, could be made to occupy any position relative to the magnets, even 

 after the entire system had been evacuated. The whole apparatus was 

 supported upon a leveling stand and packed in cotton to protect it from 

 external radiation, the thermal junction alone being exposed to the radia- 

 tion in question. 



The sensibility of the instrument used can be seen from the following 

 data : It had a full period of 8 seconds, and with a candle at a distance of a 

 meter gave a deflection of 15 cm. when the light was allowed to fall on the 

 junction after passing through a glass window. 



When the apparatus was pumped out and the radiomicrometer thus sus- 

 pended in a vacuum, the deflection for a candle at a distance of a meter was 

 50 cm. Since glass absorbs just about half the energy emitted by a candle, 

 our radiomicrometer, when provided with a rock-salt window and exposed 

 to a candle at a distance of a meter, would give a deflection of about 100 cm. 



How our instrument compared with the radiomicrometers constructed 

 and used by other investigators can be seen from the following table, taken 

 in part from the paper by Coblentz : l 



Table 1. 



The magnetic control due to small amounts of magnetic impurities in the 

 copper wire was, of course, greater the more sensitive the instrument. For 

 this reason the radiomicrometer was not used in a vacuum. The length of 

 the quartz fiber was so chosen that a candle at a distance of a meter gave a 

 deflection of 16 cm. The half period was 4 seconds. This sensibility was 

 found to be quite sufficient for work in the red and infra-red, and even for 

 wave-lengths as short as 4,500 a.u. The measurement could be carried 

 out quickly and the magnetic disturbance was practically negligible. 



1 BulUBur. Standards 4, No. 3. 



