COBLENTZ AND EMERSON I PHOTOELECTRIC SENSITIVITY 527 



was 14 by 4 mm. The radiation from the standard lamp pro- 

 duced a deflection of 13 to 17 mm. Both samples were quick 

 acting, and after exposure to light there was no lag such as 

 obtains in selenium in recovering its dark resistance. 



Selenium. A crystal of selenium, prepared by Dr. F. C. 

 Brown 4 and having a receiving surface of less than one sq. mm., 

 when exposed to the standard lamp gave a deflection of more 

 than 50 cm., which shows its great sensitivity as compared with 

 other substances. 



The mounting of the selenium crystal consisted of metal 

 electrodes between which the crystal was held by compression. 

 When operated as a photophone, by connecting the selenium 

 crystal with an audion amplifier as described on a subsequent 

 page, a loud musical note was obtained. 



Tellurium. This metal is said to change in resistance when 

 exposed to light. The present tests were made upon a mirror of 

 tellurium deposited upon a glass plate by cathode disintegration. 

 Suitable terminals were attached to a sample about 4 by 50 

 mm. No change in conductivity was observed when it was 

 exposed to light. 



Boleite. The sample of boleite [3Pb CI (OH) . Cu CI (OH) + 

 Ag CI], from Boleo, Mexico, examined was a single rectangular 

 crystal 3 by 3 by 1.5 mm. It was held by compression between 

 copper electrodes. No change in conductivity was observed 

 when the crystal was exposed to daylight or to the standard 

 incandescent lamp. 



Stibnite. Samples of this same specimen of stibnite, Sb 2 S 3 , 

 were supplied to Elliot 5 for investigation. The purpose of the 

 present investigation was to obtain a comparison of its sensi- 

 tivity with that of other substances containing antimony. 



The size of one sample examined was 4 by 7 by 0.5 mm. 

 Terminals were attached to it by heating a copper wire to in- 

 candescence in a gas flame and bringing it in contact with the 

 plate of stibnite. 



The standard carbon lamp caused a deflection of 5 cm. Stib- 



4 Browx. Phys. Rev., (2) 4:85. 1914. 

 6 Elliot. Phys. Rev., (2) 5: 53. 1915. 



