1909] 



on Electrical Striations. 



581 



the cathode C is at the end of the tube, the anode is the metal phig 

 A connected with the earth ; a very fine hole was bored in this plug 

 and through it a pencil of rays passed across the discharge and then 

 entered the side tube T. In this tube there was a screen W, covered 

 by a phosphorescent substance, in some cases willemite ; in others 

 the screen was a zinc sulphide one procured from Mr. Crlew. The 

 place of impact of the cathode rays against the screen is marked by 

 a luminous spot, and by measuring with a cathetometer the deflexion 

 of this spot the magnitude and direction of the electric force acting 

 on the cathode rays as they pass across the discharge-tube can be 

 determined. Tinfoil was wrapped round the outside of the discharge- 

 tube to neutralize the effect of electric charges on the glass walls 

 of the tube. The use of cathode rays not only avoids the disturbance 

 due to the presence of the wires, but inasmuch as the cathode rays 

 are negatively electrified particles it enables us to measure the effect 



-fH"C 



Fig. 4. 



of the field on such particles, and as it is the corpuscles which carry 

 practically all the current in the discharge, the method enables us to 

 observe in a very direct way the most important factor in the dis- 

 charge. 



The method is, however, limited to the case where the pressure 

 in the discharge-tube is low, as it is only at low pressures that the 

 cathode rays produce a well-defined spot on the screen. 



Observations made with this method showed unmistakably the 

 existence of negative forces in certain parts of tlie discharge, and in 

 fact the distribution of electric force along the tube as determined 

 by this method agreed remarkably well with that determined by the 

 method of the exploring wire. When the discharge was striated it 

 was generally found that wiiere the cathode rays passed underneath a 

 striation, i.e. on the cathode side of the bright part of the striation, 

 there was a small deflexion of the cathode rays towards the cathode 



