168 DR. FRANK HORTON ON THE DISCHARGE OF 



just before it occurs the atoms are in such a state that a small change in the electrical 

 conditions is sufficient to cause them to pass from a condition in which they are giving 

 out no light to one in which they are brightly luminous. Now the current passing 

 through the tube increases with the potential difference between the electrodes at a 

 rate which increases rapidly with the temperature of the cathode. The higher the 

 temperature of the cathode, therefore, the greater will be the effect of a given increase 

 in the potential difference, and whereas at low temperatures the appearance of the 

 luminosity may be quite gradual, the same increment in the potential difference may, 

 at high temperatures, make all the difference between no glow and a very bright one, 

 so that the luminosity appears quite suddenly. In the present experiments the gas 

 pressure was so great that the current was never saturated, but increased at an ever 

 increasing rate with the potential difference. Under these circumstances a similar 

 argument will apply to the case of the potential being kept steady and the temperature 

 being gradually increased. With a low potential difference between the electrodes the 

 luminosity would be expected to appear gradually, and with a large potential difference 

 to appear more abruptly. It has already been stated that this is what was observed. 

 The conditions which decide whether the luminosity appears round the anode or 

 the cathode need further investigation and will form the subject of future research. 



(5) Summary of Results, and Conclusion. 



The results contained in this paper may be summarised as follows : 



1. The experiments with a platinum cathode show that the negative leak from 

 platinum in helium or argon at low pressures is practically the same as in air or 

 oxygen. The variation of the negative leak per square centimetre with the 

 temperature can be expressed by an equation of the form used by H. A. WILSON and 

 by O. W. BICHABDSON, viz., x = A^e"* 472 *, where x is the current in amperes, 6 the 

 absolute temperature, and Q and A are constants. The values of these constants for 

 a cathode well cleaned with nitric acid in helium at a pressure of 3 '236 millims. 

 with a potential difference of 40 volts between the electrodes are Q = 1'22 x 10 s , 

 A= 1-55 xlO 9 . 



2. Attention has been drawn to a curious increase in the negative leak caused by 

 allowing the cathode to stand for some time with a very low gas pressure in the 

 apparatus. This increase seems to be caused by the appearance of a dark substance 

 on the surface of the platinum cathode. The substance is probably produced by the 

 action of the mercury vapour on the platinum. It could be driven away by long 

 continued heating of the cathode. 



3. The negative leak from calcium is enormously greater than from platinum at 

 the, same temperature. As with platinum, the variation of the leak with the 

 temperature can be expressed by the equation x = A0*r"**, but the observed values 

 of the negative leaks from calcium at different temperatures do not so closely agree 



