Force of the Electric Discharge through Oases. 443 



electricity carried by each discharge was very small, even 

 when the pressure of the gas was low enough to allow the 

 discharge to be appreciably deflected ; but when the pressure 

 of the gas was low and several large jars were used instead 

 of the one small one, the quantity of electricity carried by 

 each discharge through the gas was very much greater than 

 in the former case, as was shown by the galvanometer, still 

 the discharge was not visibly deflected ; while the discharge 

 carrying much less electricity in the former case, but lasting 

 a longer time, as was shown by the rotating mirror, was 

 deflected. When jars were used without any spark-gap, as 

 iu case (3) above, the first part of the discharge, which carried 

 the charge of the jars connected across the electrodes, was 

 always least deflected (see fig. 14), while the less intense but 

 more prolonged parts were always more deflected. 



As the duration of the discharge in any gas depends on 

 the pressure of the gas, the conclusion might seem to be at 

 variance with the results of previous experiments, where it 

 was shown that the deflexion was independent of the pressure 

 of the gas, provided the current in the discharge was kept 

 constant ; but in those experiments, to keep the current in the 

 discharge constant when the pressure of the gas was lowered, 

 a very large resistance had to be introduced into the discbarge 

 circuit, and this added resistance had the same effect in 

 shortening the time of discharge as the reduction of the 

 pressure had in increasing it : that adding resistance to the 

 circuit had the effect of shortening the time of discharge was 

 shown by the rotating mirror. In the experiments referred 

 to, an increase of current in the discharge was always accom- 

 panied by an increase in its duration. 



Influence of the Gas on the Deflexion of the Discharge. 

 With the explanation of the deflexion of the discharge by a 

 magnetic force given by Professor Thomson (see p. 437 above) 

 the deflexion of the discharge should be influenced by the 

 gas through which it passes in several ways : — 



(1) The Rate of Diffusion of the Gas. 

 During the interval between successive discharges the 

 dissociated or ionized gas left by one discharge scatters rapidly 

 through the tube owing to diffusion, so that the conductivity 

 along this path for the next discharge rapidly falls ; and if the 

 diffusion is very rapid, the path of least resistance for each 

 discharge may be the shortest one, i. e. the straight line 

 between the electrodes, as in the case when the frequency of 

 the discharge was made very small. 



