530 COBLENTZ AND EMERSON! PHOTOELECTRIC SENSITIVITY 



crushed while under investigation. Prolonged tests on other sam- 

 ples gave negative results as regards the production of sound. 



In view of the fact that the tests made with a sensitive gal- 

 vanometer failed to show an increase in conductivity when bis- 

 muthinite was exposed to light, it appears that the change in 

 conductivity which was observed when a certain specimen was 

 exposed to intermittent flashes of light (photophone or, rather, 

 radiophone) is the result of a thermal change within the crystal, 

 or perhaps a change in the contact resistance at the electrodes. 7 

 In this connection the following experiments on thin strips of 

 metals are of interest. 



Platinum, and gold. In conclusion it is of interest to record 

 the results obtained when using thin blackened strips of plati- 

 num and of gold-leaf as radiophones, by connecting them through 

 a battery to an amplifier. 



These blackened strips were warmed intermittently by expos- 

 ing them through a rotating sectored disk to the acetylene 

 flame, as already described. 



When a sensitive platinum bolometer receiver was used as a 

 radiophone, the sound produced in the telephone was not very 

 audible. This no doubt was due to the great heat capacity of 

 the material which prevented the rapid alternations in resistance, 

 and hence in electric current, from being of sufficient magnitude 

 to affect the telephone receiver. 



Using a lightly smoked strip (6 by 2.5 mm.) of gold-leaf, the 

 ends of which were clamped between thin (0.02 mm.) strips of 

 tin, the sound produced in the telephone receiver was as loud as 

 was observed in the photophone made of selenium. 



This device was mounted in a glass bulb which could be evacu- 

 ated. As was to be expected, there was no marked difference in 

 the intensity of the sound produced when operated in air and 

 in a vacuum. 



In the gold-leaf radiophone as used, the limit of audibility 

 was attained for a light (radiant power) intensity of 4.8 X 10 ~ 5 



7 It would be interesting to determine whether the effect is dependent upon 

 the axial direction of exposure. In the present case the needle-crystals were 

 parallel to the electrodes. 



