500 .Prof. Skinner on Conditions controlling the Drop of 



the walls of the tube. It increases then very rapidly with 

 decreasing gas-pressure, so that with a source of limited P.D. 

 it soon prevents the passage, of a current through the tube. 

 Under these conditions the tube emits a hissing tone when 

 the current is passing, and the cathode surface rapidly dis- 

 integrates, as shown by the deposit on the walls of the tube. 

 If the current be reversed so that the disk serves as cathode, 

 the presence of the tinfoil does not produce the slightest effect. 

 These phenomena may be explained b}- assuming a con- 

 denser-effect in which the cylinder cathode surface and the 

 tinfoil surrounding the tube are the two surfaces of the 

 condenser. With unconfined negative glow the highly con- 

 ducting gas screens off the electrostatic forces which would 

 otherwise act between the condenser-surfaces. When, how- 

 ever, by confining the glow the ionization is reduced, the 

 condenser-effect enters, increasing the velocity with which 

 the positive ions move up to the cathode, and thereby in- 

 creasing their resistance to discharge causes for the same 

 terminal P.D. a decreased current, as observed. With the 

 current reversed the negative glow is not only unconfined in 

 its expansion, but the condenser-effect acts on the ions moving 

 up to the anode instead of the cathode, either of which would 

 prevent any observable change due to the presence of the 

 coating. 



Effect of a Magnetic Field on the Electrode Drop, 

 The effect of a magnetic field on the discharge has been 

 studied by different observers. The following experiments 

 were made to gain a more definite knowledge of the effect of 

 a magnetic field on the electrode drop. 



A tube of form shown in fig. 7 was used. A disk-electrode 

 A, of aluminium, was inlaid in guttapercha. The drop at 

 this could be obtained by means of an adjustable wire W. 

 A ponderous magnet being used, the tube was provided with 

 a ground-joint T, so that without shifting it the electrode 

 could be rotated to any desired direction with respect to that 

 of the magnetic field. 



At the Cathode, — In Table III. the effect of the field on 

 the cathode drop (the P.D. between the cathode and the nega- 

 tive glow) is given, and the results plotted in fig. 8 (p. 502). 

 They are independent of the direction of the field, and were 

 taken at those pressures at which the field appears to have no 

 influence on the distribution of current at the cathode. At 

 a pressure of 1*8 mm. a field of 760 lines per sq. cm, pro- 

 duces an inappreciable effect ; at 0*5 mm. a field of 650 lines 

 reduces the drop about 8 volts : at 0*2 mm., 20 volts : at a 

 much lower pressure about 200 volts. The decrease seems 



