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



bright, well-defined ring passes through the bulb near to the 

 surface of the glass, the gas inside this ring being, as far as 

 can be judged, quite free from any discharge. If now a bulb 

 whose diameter is less than that of the luminous ring is 

 inserted in the primary in place of the larger bulb, a bright 

 ring will start in this, though at this distance from the pri- 

 mary there was no discharge in the larger bulb. Thus when 

 the large bulb was in the primary, the discharge through its 

 outer portions screened the interior from electromotive forces 

 to an extent sufficient to stop a discharge which would other- 

 wise take place. 



The screening action of these discharges is also shown by 

 the following experiment. A, B, C, 

 fig. 11, is the section of a glass vessel 

 shaped like a Bunsen's calorimeter ; in 

 the inner portion A, B, C of this vessel 

 an exhausted tube is placed, while a 

 pipe from the outer vessel leads to a 

 mercury pump and enables us to alter 

 the pressure at will. The primary coil, 

 L, M, is wound round the outer tube. 

 When the air in the outer tube is at 



Fig. 11. 



r~\ 



r 



> 



B 



j^*-Toj)ump 



atmospheric pressure, the discharge 

 caused by the action of the primary 

 passes in the tube E inserted in A, B, C ; 

 but when the pressure in the outer tube 

 is reduced until a discharge passes 

 through it, the discharge in the inner 

 one stops ; the discharge in the outer 

 tube has thus shielded the inner tube from the action of the 

 primary. If the exhaustion of the outer tube is carried so far 

 that the discharge through it ceases, that in the inner tube 

 begins again. It requires very high exhaustion to do this, and 

 as on account of the joints it is unsafe to make the vessel very 

 hot during the pumping, I have found it impossible to keep a 

 vacuum good enough to show this effect for more than from 

 half to three quarters of an hour; in that time sufficient gas 

 seems to have escaped from the sides of the vessel to make 

 the pressure too high to show this effect, and it then takes 

 from two to three hours' pumping to get the tube back again 

 into its former state. An interesting feature of this experi- 

 ment is that for a small range of pressure, just greater than 

 that at which the discharge first appears in the outer tube, 

 there is no discharge in either of the tubes ; thus the action 

 of the primary is screened off from the inner tube, though 

 there is no luminosity visible in the outer one ; this shows 



