hij the ^lotion of J^osidrc Ions in a Gas. 



017 



In (letormining the potentials experimentally, the light 

 from the external source should be of small intensity since 

 large currents would oive rise to effects which are not taken 

 into consideration in the theory. A large current in a gas 

 tends to reduce the resistance and disturbs the uniformity o£ 

 the electric field between the electrodes. Thus the potential- 

 difference required to maintain a discharge in a gas is much 

 less than that which is necessary to start a discharge in a gas 



FiiT. 5. 



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which had been acting as an insulator. If the intensity of 

 the light is very strong, the current might be sufficient to 

 break dowm the insulation when the quantity a— /3e^"~^' 

 is small, but does not vanish. The currents used in the ex- 

 periments were not large enough to produce these effects, 

 since the ratio of the currents for the different distances 

 between the plates did not alter with the intensity of the 

 light. 



The spark which takes place when no radiation is falling 

 on the electrodes may be attributed to the small number of 

 ions which, as Elster and Geitel* and C. T. R. Wilson t have 

 shown, are always present in gases. These would eventually 



• Phifsikalische Zeitschrift, ii. .Tahrgang, Xo. 8, Nov. 24, 1901. 

 t iW. Ko^-. Soc. vol. Ixviii. (1901). 



