VACUUM RADIOMICROMETER. 



123 



cm., and about 8 mm. in diameter. The area of the thin copper vane 

 was 3 sq. mm. It was blackened by covering it with a little Canada 

 balsam, upon which was dusted fine copper oxide. The weight of the 

 complete loop was less than 10 mg. The magnet was taken from a 

 Weston ammeter. It was used without pole pieces, since the latter 



FIG. 92. Radiomicrometer, about two-thirds actual size. 



caused too much damping. The top was covered with the bulb of a 

 thistle-tube, the mouth of which was ground flat. The heavy metal 

 base of the radiomicrometer is of Swedish iron. The narrow vertical 

 tube is of brass. All the joints are made with Kohtinsky cement, except 

 the one at the top and the one containing the support of the rock- 

 salt window. The top seal may be made with mercury. A torsion head 



