170 DISCHARGE OF NEGATIVE ELECTRICITY. 



luminosity appears abruptly if the temperature of the cathode is high and the 

 potential difference between the electrodes is gradually increased, or if a large 

 potential difference is used and the temperature of the cathode is gradually raised. 

 The abrupt appearance is accompanied by a large increase of the current passing 

 through the tube. The luminosity cannot be obtained with a potential difference of 

 less than 18 volts, which is probably the value of the anode fall of potential. With 

 small differences of potential (between 18 and 40 volts) the luminosity appears quite 

 gradually as the temperature of the cathode is raised, and without any sudden 

 increase in the current passing. 



With low potential differences the luminosity appears sometimes round the anode 

 and sometimes round the cathode. In the former case it may be caused to leave the 

 anode and to appear round the cathode by increasing the potential difference. This 

 inversion of the appearance of the discharge is always accompanied by a large increase 

 in the current. 



In conclusion, I wish to say that my best thanks are due to Professor J. J. THOMSON 

 for his advice and interest in these experiments, which were carried out at the 

 Cavendish Laboratory. 



