[LEVI] PHOTO-ELECTRIC CONDUCTIVITY OF DIAMOND 245 



This instability, which appears with time and with high voltage, 

 corresponds to the component of the photo-electric current named 

 by Gudden and PohP the secondary current. It occurs irrespective of 

 the direction of the current through the crystal, and its appearance 

 substantiates the hypothesis of Gudden and Pohl that conduction 

 takes place by means of negative carriers which, becoming separated 

 from netural atoms, leave them positively charged. The positive 

 ions then create a space charge within the crystal, with consequent 

 establishment of strong counter fields. The latter may become 

 strong enough to overbalance the external field, in which case the 

 current in the crystal will flow in a direction opposite to that due 

 to the external field. Upon the demolishing of the counter fields 

 with the passage of this current, the influence of the external field 

 becomes predominant. Such a hypothesis would account for the 

 surging back and forth of the current, which was observed after 

 lengthy exposure of the crystal to light, in a strong field. 



« 



(g) Effect of Wave-length of Exciting Light. 



These irregularities made it necessary to exercise care in taking 

 readings of photo-electric currents at various wave-lengths. To 

 obtain uniformity, the crystal was illuminated for exactly 15 seconds 

 for every reading, and the maximum deflection attained in that time 

 was taken as the value of the photo-electric current. After shutting 

 ofï the light, the applied potential was removed for several minutes, 

 so as to bring the crystal back to its original state for every reading. 

 With these precautions, steady and consistent results were obtained. 



The curve in Fig. 3 is an example of the results obtained. The 

 abscissae represent wave-length in Angstrom Units, and the ordinates 

 photo-electric current per unit of light energy per second. From the 

 figure, it is seen that diamond No. 1 is increasingly sensitive with 

 decreasing wave-length, the maximum sensitivity being at X = 2536 A. 

 By comparison with the work of Gudden and Pohl^ it may be con- 

 cluded, from the deviation from a smooth curve in Fig. 3, that diamond 

 No. 1 contains impurities. 



Repeated measurements gave the same characteristic curve, 

 regardless of the direction of the external field and the magnitude 

 of the applied voltage. Variation of these conditions changed the 

 value of the photo-electric current, and hence of the ordinates in Fig. 3, 

 without affecting the character of the curve. 



iQudden and Pohl, Zeitschr. fur Physik, 6, p. 249; also 7, p.' 65, 1921. 

 ^Gudden and Pohl, Zeitschr. fur Physik, 3, p. 125, 1920. 



