1903.] Photo-electric Discharge from Metallic Surfaces. 11 



The same product is formed on treating the acid with phosphorus 

 trichloride. Methylstannichloroform crystallises from light petroleum 

 in long colourless prisms melting at 105107 and distils without 

 decomposition at 179180. It fumes in the air, dissolves to a 

 clear solution in water, and is very soluble in the ordinary organic 

 solvents. 



"On the Photo-electric Discharge from Metallic Surfaces in 

 different Gases." By W. MANSERGH VARLEY, M.Sc, (Viet), 

 Ph.D. (Strasburg), 1851 Exhibition Research Scholar, 

 Emmanuel College, Cambridge. Communicated by Professor 

 J. J. THOMSON, F.R.S. Received April 24, Read May 14 

 1903. -V 



(Abstract.) 



The object of the experiments described in this paper was to study 

 as systematically as possible the effect of the pressure and nature of 

 the gas with which a metal surface is surrounded upon the magnitude 

 of the photo-electric current from the surface. 



On account of the complicated nature of the relation between the 

 photo-electric current and the potential difference between the 

 electrodes, it is not enough to state, for example, that the current in 

 air at 760 mms. pressure is so many times that in air at 50 mms. 

 pressure, without specifying the exact conditions under which the 

 observations were taken. The method used in these experiments was 

 to draw the complete curves connecting the current and the potential 

 difference at each pressure examined, keeping the intensity of the ultra- 

 violet illumination and the other conditions unaltered. 



Great difficulty was encountered in finding a suitable source of ultra- 

 violet light which would remain constant in intensity while long series 

 of observations were being taken, but ultimately the spark between 

 iron terminals in an atmosphere of pure dry hydrogen was found to 

 answer excellently. The spark gap was in parallel with three Leyden 

 jars in the secondary circuit of an induction coil, used as a trans- 

 former. 



The photo-electric currents were measured from a metal usually 

 zinc surface placed a few millimetres behind a fine gauze, through 

 which the light passed, and which served as the positive electrode. A 

 brass vessel, with a quartz window to admit the light, served to 

 contain the electrodes. It was connected to pump, gauge, &c., so that 

 the pressure or gas could be changed at will. 



A second similar apparatus was used as a control for the intensity 

 of the light. 



