430 Prof. Richardson and Dr. Bazzoni : Experiments 



pressure of the mercury rises. These statements are illus- 

 trated by the following numbers, in which the values of the 

 vapour pressure of the mercury present are very rough 

 guesses from the temperatures as indicated by a thermometer 

 in contact with the bulb : — 



Pressure of 

 Kg Vapour. 



Potential 

 at A. 

 volts. 



Current 

 at A. 



jinicroamps. 



Potential 



at A l . 



volts. 



Current 



at A r 



mioroainps. 



Heating 



Current. 



amps. 



0-0001 cm. 



40 



406 



36 



1188 



105 



0001 



26 



120 



21 



792 



1-04 



- 0001 



23 



198 



135 



3432 



1-12 



0-05 



125 



36 



11-9 



180 



1-05 



10 



11-7 



S 22o 



11-7 



223 



1-07 



It will be seen from the numbers in the second and third 

 rows that increasing the filament tempeiature tends to diminish 

 the minimum potential necessary to maintain the currents on 

 the high-level range. This is shown also by the following 

 numbers : — 



Vapour Pressure, 

 cm. of Hg 



Minimum Voltage 

 for A 1 . 



Heating Current. 



Current across 



AB — * C. 



about 



VultS. 



amps. 



nucroamps. 



(A) <001 



18-5 



1-05 



1584 





14 



1-09 



2700 





13-5 



1-10 



2700 



(B) >001 



15 



1-05 



1089 





13 2 



109 



2838 





13 



110 



3036 



(C) <-05 



121 



105 



192 





120 



1-10 



252 



(D) >-05 



120 



105 



210 





11-9 



1-10 



200 



1-0 



11-7 



107 



230 



The observations under A and C were obtained imme- 

 diately after taking B and D respectively, and then opening 

 the connexion to the charcoal tube. The pressures in A and 

 C are therefore believed to be respectively somewhat less than 

 those for B and D. 



These and other observations indicate that increasing the 

 pressure of the mercury vapour or the temperature of the 

 filament does not lower the potential necessary to maintain 



