202 



Professor Joseph John Thomson 



[Jan. 19, 



In a discharoje tube conveying- an electrical current, the amount of 

 work per unit volume of the .i^as done by the electrical forces per unit 

 time varies very largely from one point of the tube to another. If the 

 cross section of the discharge is the same at all parts of the tube, so 

 that the current density is uniform, the rate at which the electrical 

 forces do work will be proportional to the electric force. As this is 

 much greater near the cathode than at other parts of the tube, we 

 should expect the lines of systems of the type B to preponderate near 

 the cathode, and to be absent or much feebler in other parts of the 

 tube. If the tube were of the type frequently used for spectroscopic 

 purposes, with a capillary portion in the middle, then, since the current 

 density is much greater in this portion than in any other, the rate of 

 work per unit volume of the gas will be much greater in the capillary 

 portions than in the wide parts of the tube, and we should therefore 

 expect the lines of systems of the type B to be much more prominent 

 in the capillary part than in the wide part. 



Fig. (I. 



Effect of Self-induction and Capacity. — Suppose that we have a 

 tube of uniform bore arranged as in Fig. 6, the terminals of the 

 tube being connected with tlie plates of a condenser of capacity C, 

 and tliat there is a coil whose coefficient of self-induction is L, placed 

 in series with the tube ; tlien if the discharge through the coil begins 

 wlien the potential difference between the plates of the condenser is 

 \'o, the potential difference between the plates after a time t will be 



Vo cos p t 



and the cnrreiit tiiroiigli the tul)e 



C Vo /? sinji)^ 



where j) = 



VLC 



