Electric Intensity and Conductivity in Rarefied Gases. 509 



Fig. 3 is for the commonest type of-discharge with a uni- 

 form positive column. At each end of the positive column 

 the electric intensity rises slightly, and then falls to a small 

 value. The rise before the fall at the positive end of the 

 positive column is often very considerable. The following 

 numbers are for a discharge in air at 0*5 mm. pressure with 

 current 0*28 milliampere. 



Distance from Electric 



Positive Electrode. Intensity, 



cms. volts per cm. 



l'« 21 



1-7 19 



1-2 23 



0-6 27 



0-35 32 



0-24 33 



0-16 21 



0-08 



At pressures above 1 millim. the fall in the intensity near the 

 positive electrode occurs so near to the positive electrode that 

 it is difficult to observe; but 1 do not think there is any reason 

 to doubt that it still exists. 



The " positive drop " at the surface of the positive electrode 

 measured by Skinner (loc. cit.) was always noticed in these 

 experiments when one of the exploiing-wires came into 

 contact with the positive electrode. It appears, as Skinner 

 showed, to occur excessively close to the surface of the elec- 

 trode, near which the intensity is zero or even apparently 

 negative. 



That the electric intensity near the positive electrode can 

 be really negative seems unlikely, and it is consequently 

 probable that the exploring-wires, when near the positive 

 electrode, either fail to take up the potential of the surrounding 

 gas, or else that the wires so disturb the discharge that their 

 presence converts the very small positive intensity into a 

 negative intensity at the place where the wires are. To 

 obtain information on this point some experiments were tried 

 in which the exploring- wires in the apparatus were replaced 

 by fine platinum tubes from which a stream of mercury 

 could be allowed to flow, breaking up into fine drops 

 travelling two or three millimetres horizontally. It was 

 thought that the jet of mercury would act as a " mercury 

 dropper," and so take up the real potential of the surrounding 

 gas. The platinum tubing used was 05 millim. external 



