222 On Certain Types of Electric Discharge. 



high ; for the electric intensity near the cathode, in this case, 

 being great, the energy required by a corpuscle for ionizing 

 the gas will be acquired at once. 



21. As the pressure decreases, the electric intensity near 

 the cathode decreases also, and the corpuscle has to move 

 through a finite distance under the action o£ the electric 

 force before it acquires sufficient energy for producing 

 ionization. The space through which a corpuscle shot off from 

 the cathode has to traverse, before it is in a condition to ionize 

 the gas, would be the Faraday dark space. The energy so 

 acquired, however, is used up in producing ionization in the 

 first illuminated area. A corpuscle issuing from this 

 illuminated area has, therefore, to move through some 

 distance before it is again in a condition to ionize the 

 gas. But as the average electric force at the striae is 

 greater than along the Faraday dark space (H. A. Wilson, 

 Proc. Camb. Phil. Soc. xi.) a corpuscle has to move 

 through a much shorter distance than the Faraday dark 

 space, in order to recover the minimum energy required (Phil. 

 Mag. Feb. 1920) for ionization. Accordingly, the dark 

 space along the striae is of shorter length than the Faraday 

 dark space. 



22. When a radial magnetic field is introduced, a 

 corpuscle issuing from the cathode tends to move under the 

 electric force along a line of discharge and undergoes at the 

 same time an angular displacement under the magnetic force. 

 The resultant velocity acquired attains to the minimum value 

 required for ionization at a shorter distance from the cathode 

 than when the magnetic field was not on. The effect of a 

 magnetic field is, therefore, to shorten the Faraday dark space 

 and all the other dark spaces and, pari passu, to increase the 

 number of strice. Moreover, the ions in the striatory dis- 

 charge, under the joint effect of electric and magnetic fields, 

 move in a spiral, the tangent to the spiral at any point being 

 inclined to the axis of the tube, one way or the other, 

 according to the direction of the magnetic lines of force in 

 the field. Remembering that the movement of ions under the 

 electric field itself produces a magnetic field (necessarily 

 comparatively weak in comparison with the extraneous field 

 th;it may be introduced), we conclude that ions in a striatory 

 discharge in general move in spirals, more or less inclined to 

 the axis of the tube, even without the application of a 

 magnetic field. 



Measurements are being undertaken for a quantitative 

 verification of the above theory. 



Our thanks are due to the authorities of the Presidency 

 College, Calcutta, for facilities for carrying on our work there. 



