through Exhausted Tubes without Electrodes. 453 



surrounded by the primary coil C, D. B was exhausted and 

 then sealed off, while A was left connected to the pump. 

 When A was at atmospheric pressure a bright discharge took 

 place in B outside A; on pumping A a stage was reached in 

 which no discharge could be seen in either A or B. On 

 letting air into A the discharge appeared again in B ; on 

 pumping A still further a discharge appeared in A, but not 

 in B. The appearance presented by the discharge round the 



Efr. 13. 



bulb A (filled in this case with air at high pressure) is very 

 remarkable. At the highest pressure at which the discharge 

 passed it took the form of a thin ring round the middle of A; 

 as the pressure got lower and lower the discharge broadened 

 out, and at very low pressures formed for the greater part of 

 its course two separate rings which ran together in the space 

 between one side of the sphere and the tube. 



On the Effect produced by Conductors near the 

 Discharge -tube. 



The intensity of the discharge is very much affected by 

 the presence of conductors in the neighbourhood of the dis- 

 charge-tube, especially conductors which have large capacity 

 or which are connected to earth. Let us take, for ex- 

 ample, a very simple case, that of a bulb surrounded by a 

 primary which is connected to earth ; in this case the ap- 

 proach of the hand, or any conductor connected to earth, will 



and at the same time less well- 



make the discharge 



brighter 



