on the Discharge of Klectricity through Vacuum-Tube, 69 



case, therefore, only a fraction of the total volts across the 

 ends of the vacuum-tube was measured. 



A large number of measurements were made with the 

 cathode at different temperatures, keeping the pressure of the 

 gas and the strength of the magnetic field constant. As 

 the cathode was gradually heated, the reduction in the volt- 

 meter-reading was small at first and altered only slowly, but 

 when a certain temperature was reached a very slight increase 

 in the cathode temperature produced a large fall in the 

 voltmeter-reading. When the cathode was only heated 

 sufficiently to produce a comparatively small decrease in the 

 voltmeter-reading, it was found mat turning on the magnet 

 decreased the potential-difference between the ends of the tube 

 to a considerable extent, as in Dr. Willows' experiments. 

 Upon making the cathode hotter, the fall of potential pro- 

 duced by the magnet became* smaller until it was reduced to 

 zero and finally became a rise. The following is a typical 

 series of measurements : — 



Pressure. 





D. 



M. 



1 



0-05 ram. 

 0-05 „ 

 005 „ 

 0-05 „ 



Cathode at dull red heat ... 

 Cathode somewhat hotter ... 

 Cathode bright red heat 

 Cathode whi te hot 



21H 

 41 



63 



Drop 11-6 

 Drop 10 6 

 Drop 4*5 

 Else 28-6 





Where D = Percentage drop in voltmeter-reading caused by 

 heating the cathode ; 



and M = Percentage alteration in voltmeter-reading pro- 

 duced by turning on the magnet, the cathode 

 still being hot. 



These results are evidently in agreement with the theory 

 stated above, for they show that the decrease in the resist- 

 ance caused by a magnetic field at the cathode depends on 

 the existence of a large cathode-fall of potential, and that 

 when this cathode-fall of potential is sufficiently reduced a 

 transverse magnetic field produces an increase in the resist- 

 ance here, just as it does in other regions of the tube. 



The change in the magnitude of the effect with variations 

 in the strength of the magnetic field was also investigated. 

 The pressure of the gas and temperature of the cathode were 

 kept constant while the current passing round the magnetizing 



