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ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1923 



thin, narrow, mica strips, V 19 V 2 . Immediately in front of V, 

 and directly behind V 2 are mounted still narrower mica strips, P, 

 coated with very thin deposits of metallic platinum. When electric 

 waves fall on metal conductors, they cause very small oscillatory 

 currents in the surface layer of the metal. Such thin metal coatings 

 as P possess high electrical resistance. Hence these oscillatory 

 currents generate minute quantities of heat, which in these experi- 



TO VACUUM 

 GAUGE 



□ M 



t> 



v 



# 



D 



V 2 



*-? 



Fig. 2. — Sketch of electric wave receiver 



ments raise the temperature of P by amounts roughly of the order of 

 one-millionth of a degree. The resulting increased gas pressure on 

 the warmed metal sides of the vanes V x and V 2 tend to rotate the 

 system in the direction shown bj 7 the arrows. The suspension also 

 carries a very thin glass silvered mirror, M, by which the amount 

 of the rotation of the system can be accurately measured. This 

 suspension complete weighing usually less than 1 mg. (about one- 

 thirty thousandth part of the weight of a 2-cent letter) is hung on a 

 very fine fiber of spun quartz, F, from a central stem in the double- 



