620 



Prof. Skinner on the Drop of Potential 



conditions, as do H. A. Wilson's curves of cross-conductivity*. 

 While the constant potential-gradient of the positive column 

 decreases through the Faraday dark space to a minimum 

 value in the negative glowf, and (as will be shown) may 

 possess a zero value also in the cathode dark space, Wilson 

 found the cross-conductivity of the gas greater in the luminous 

 positive column than in the Faraday dark space, greatest in 

 the negative glow, and very tow in the cathode dark space. 

 The drop at the anode being smaller in the luminous positive 

 column than in the Faraday dark space, least in the negative 

 glow, and greatest in the cathode dark space, we conclude 

 that the conditions which increase the cross-conductivity of 

 the gas at the anode, lower its drop. If the cross-conductivity 

 depends simply on the degree of ionization, then, other con- 

 ditions being the same, the drop at the anode is lowered by 

 increased ionization. This is in harmony with Professor 

 J. J. Thorns m's theory, that the drop at the anode is necessary 

 to ionize the gas in its immediate vicinity J. If the gas be 



Table III. 



Drop of Potential at Electrodes with Distance apart. 



Gas-pressure 2*0 mm. Current 1*0 m.a. 



Distance in 



Millimetres. 



Drop at Anode 



in Volts. 



Drop at Cathode 

 in Volt 8. 



Drop between 

 Electrodes in Volts. 



10 







265 





6 







265 





3-5 







265 





2-5 



45 





275 





1-5 



75 





485 



560 



ionized from some external cause the drop should be reduced, 

 or should vanish, according to the degree of ionization. The 

 question however arises : Why is the ionization less in the 

 Faraday dark space than in the luminous positive column, 

 while the potential gradient is greater in the latter ? In the 

 case of the anode drop, it cannot be due to an additional 

 ionizing effect of the luminosity in the positive column over 

 that in the dark space, as suggested by Wilson to explain the 

 difference in cross-conductivity. For, as stated above, the 



* H. A. Wilson, Phil. Mag. June 1900, p. 505. 



t See W. P. Graham, Wied. Ann. lxiv. p. 69 (1898). 



% J. J. Thomson, Phil. Mag. April 1901, p. 361. 



