538 COBLENTZ AND KAHLER : PHOTOELECTRIC SENSITIVITY 



lamp. A similar spectral photo-negative response was observed 

 in some samples of stibnite,- when exposed to radiations of wave- 

 lengths less than X = 0.657 M- But heretofore no one appears 

 to have observed that, for wave-lengths less than about 0.65 n, 

 the nature of the photoelectric response depends upon the voltage 

 applied to the substance under test. 



We have found in some samples of molybdenite that, for the 

 visible spectrum extending to about X = 0.647 M, the electric 

 response is photo-positive or photo-negative, depending upon 

 the voltage applied to the terminals of the receiver. For wave- 

 lengths greater than X = 0.647 M the photoelectric response was 

 observed to be photo-positive whatever the applied voltage. 



The region of transition in the spectrum, in which the action 

 changes from photo-negative to photo-positive, is very narrow 

 — less than 9 AU. 



The critical voltage is very small, as may be inferred from the 

 fact that an increase of 1.3 volts (additional dry cell) changed a 

 positive-negative galvanometer deflection of ± i cm. into a 

 purely negative deflection of — 24 to — 26 cm., which is the 

 photo-negative response under discussion. 



There seem to be two contending forces acting. The one which 

 causes the photo-positive respons:^ acts quickly and prevails on 

 low voltage. The photo-negative action builds up more slowly 

 and is predominant on high voltages. As a result of these two 

 forces, for certain applied voltages, on exposing the molybdenite 

 receiver to light of wave-lengths less than 0.647 M. the galvanom- 

 eter deflection is first positive, then decreases in value (and 

 may even become negative) when, on shutting off the light 

 stimulus, there is a further deflection in the negative direction, 

 after which the deflection returns to the original zero scale read- 

 ing. 



For example, using the wave-length X = 0.5876 /j. as a light 

 stimulus, and applying a potential of 10 volts, the galvanometer 

 deflection was almost entirely positive. On 20 volts the de- 

 flection was partly positive and partly negative. On 29 volts 

 the positive response was almost eliminated, and the negative 



2 Elliot, Phys. Rev. 5: 62. 191 5. 



