452 Prof. J. J. Thomson on the Discharge of Electricity 



electrolyte, which produces an equal effect in stopping the 

 discharge, depends upon the length of spark in the primary 

 current, and so upon the electromotive force acting upon the 

 air. The longer the spark the greater is the molecular con- 

 ductivity of the air in comparison with that of the electrolyte. 

 This indicates that the conduction through the air does not 

 follow Ohm's law. This is what we should expect, as under 

 large electromotive forces more molecules are split up and 

 take part in the conduction of the electricity. This great 

 conductivity of rarefied gases in those cases where the elec- 

 tricity has not to pass from metal &c. into the gas are in 

 striking contrast with the infmitesimaliy small values for the 

 same property which are deduced from experiments on tubes 

 with electrodes. 



I was first led to suspect this high conductivity for rarefied 

 gases by observing the appearance presented by the ring-dis- 

 charge in bulbs ; the ring, unless the pressure is exceedingly 

 low, ceases at a distance of little more than 1 centim. from 

 the surface of the bulb, this thickness of conducting gas being 

 sufficient to screen oft' the electromotive intensity from the 

 interior. From experiments which I had made on the screen- 

 ing effect of electrolytes (Proc. Poy. Soc. xlv. p. 269), I knew 

 that it would require a very strong solution of an electrolyte 

 to produce screening comparable with this. To compare the 

 screening effects more directly than by the method just 

 described 1 tried the following experiment. The discharge- 

 tube, fig. 11, was pumped until the discharge passed through 

 it very freely; an exhausted tube was then pushed down the 

 central opening, it remained quite free from any visible dis- 

 charge; the primary was now wound round a cylinder of the 

 same diameter as the discharge-tube of fig. 11, and this 

 cylinder was filled with distilled water. When the tube, which 

 had previously remained dark when placed in the exhausted 

 discharge-tube, was immersed in the water, a brilliant discharge 

 took place in it; and it was necessary to add about 25 per 

 cent, of sulphuric acid to the water before the shielding effect 

 of the mixture was sufficient to keep the tube dark. This 

 experiment shows perhaps even more directly than the other 

 the great conductivity of a rarefied gas under large electro- 

 motive forces when nothing but the gas is in the way of the 

 passage of the current. 



An experiment made in this connexion illustrates the re- 

 mark made before as to the large effects produced by dis- 

 charges through the gas which are not accompanied by 

 luminosity. A bulb A was fused into a tube B which was 



