374 



Dr. Willows on the Action of a Magnetic 



•002 mm., a rise in volts was brought about by the field. 

 The results, however, at the lowest pressures were very 

 irregular, and it may be that Almy's supposition as to the 

 concentration of the discharge may have some weight here. 



When the transverse field is applied, the Crookes dark 

 space is greatly shortened on that side of the tube to which 

 the discharge is deflected, very little alteration in its length 

 is seen on the other side. The glow at the surface of the 

 cathode (called by German writers the first negative layer) 

 is also affected. The figures adjoining (fig. 2) show this. 



Fiff.2. 



VE 



Fig. 3. 



WITHOUT MAGNET WITH MAGNET 



When the field is off, the first negative layer, A, covers the 

 centre only of the cathode and is cylindrical in section ; with 

 the field on, its length is considerably less and it covers much 

 more of the cathode area. 



Since this layer is supposed to be due to ionization caused 

 by canal-rays striking the cathode, 

 an experiment was performed to see 

 whether the path of these rays was 

 altered by the field. A tube, as 

 shown in fig. 3, with an aluminium 

 wire projecting across the surface of 

 the cathode was used. 



The surface of the cathode behind 

 this w T ire is devoid of all glow as 

 shown by various investigators. The 

 magnet caused no shift of this shadow, 

 so that the fields used were not suffi- 

 cient to deflect the canal-rays. 



Townsend * has given a complete 

 explanation of the fact that there 

 exists a pressure for which the 

 sparking potential between two fixed 

 electrodes is a minimum. The fact 

 that there exists a pressure for which 

 the E.M.F. required to maintain the 

 discharge is a minimum yet awaits a 

 detailed explanation. The following 

 observation on this point is of interest. 



* Phil. Mag. Nov. 1903, p. 598 



