COBLENTZ AND EMERSON: PHOTOELECTRIC SENSITIVITY 531 



watts. Using a larger receiver and amplifier and a larger cur- 

 rent (which was 0.2 amp. in the present tests) through the 

 receiver, the sensitivity could be greatly increased. 



summary » 



This paper summarizes the results of an investigation of 

 various substances (1) for an increase in electrical conduc- 

 tivity caused by the action of light upon them, and (2) for 

 electrical discharging activity when they were charged to a 

 negative potential in an evacuated chamber and exposed to 

 light. 



Pure gallium and silver sulphide were found to have but small 

 photoelectric discharging action when charged to a negative 

 potential and exposed to light. 



No change on exposure to light was observed in the electrical 

 conductivity of tellurium, boleite, bismuthinite, and mixtures of 

 the sulphides of lead and antimony. 



An increase in electrical conductivity on exposure to light was 

 observed in crystals of selenium, stibnite, boulangerite, jameso- 

 nite, and silver sulphide. 



Experiments are described in which crystals of bismuthinite 

 were joined through a battery to the grid circuit of an audion 

 amplifier and a telephone. A change in current in this circuit 

 affected the telephone. The light stimulus was interrupted by 

 means of a rotating sectored disk, as used in Bell's selenium 

 photophone. When using a cell or crystal of selenium the 

 fluctuations in light intensity produced a sufficient change in 

 conductivity to cause a musical note in the telephone. Simi- 

 larly, in several samples of a crystal of bismuthinite a change 

 in conductivity was produced, which caused an audible sound 

 in the telephone. However, from various tests it is believed 

 that this is not a true photoelectrical change (increase) in con- 

 ductivity, but is due to a thermal resistance change within the 

 crystal or to a change of resistance at the point of contact of the 

 crystal with the metal electrodes between which the crystal was 

 held by compression. 



