526 COBLENTZ AND EMERSON I PHOTOELECTRIC SENSITIVITY 



SUBSTANCES EXAMINED 



Gallium. The material examined was the highly purified 

 metal prepared and supplied by Dr. H. S. Uhler. This metal 

 was solid, thus differing from the impure material, which is a 

 liquid. It was melted and solidified over a platinum wire sealed 

 into a glass bulb, thus forming the negative electrode of a photo- 

 electric cell from which the air was exhausted. The anode was 

 a loop of platinum wire, situated at a distance of about 12 mm. 

 above the gallium electrode. 



A potential of 340 volts was applied to the cell which was 

 connected to a sensitive iron-clad Thomson galvanometer 

 (i = 5 X 10 ~ 10 amp.). 



The results obtained proved disappointing, this metal being 

 quite insensitive to light. When the cell was exposed to day- 

 light the photoelectric current produced a deflection of only 4 to 

 5 mm., whereas similarly exposing a potassium photoelectric cell 

 the photoelectric current was sufficient to give a deflection 

 beyond the range of the scale. 



Silver sulphide. The sample examined was a thin flexible 

 strip, 6 by 10 mm. in area, prepared by Mr. G. W. Vinal. 3 In 

 one test the silver sulphide formed the negative electrode of a 

 photoelectric cell (evacuated glass bulb about 5 cm. diameter 

 with a ring of platinum wire for the anode) similar to the gallium 

 cell just described. It was connected through an iron-clad 

 Thomson galvanometer to a battery of 340 volts. When ex- 

 posed to daylight a deflection of perhaps 1 to 2 mm. was observed, 

 but no deflection resulted from exposure to the standard carbon 

 lamp. 



In the second test, copper wires were melted to the ends of a 

 strip (3 by 5 by 0.3 mm.) of silver sulphide which was con- 

 nected in series with a high resistance, a storage cell of 2 volts, 

 and a d'Arsonval galvanometer. When exposed to the standard 

 lamp, the galvanometer deflection was 10 mm. In another 

 sample about 2 cm. long, the ends joining the copper wires were 

 covered to prevent thermoelectric currents. The exposed area 



3 Vinal. Bur. Standards Scientific Paper No. 310. 



