Difference of Potential and Thermionic Emission. 559 



corresponds to a change of 0*71 volt in the potential difference 

 between cathode and anode, and is in such a direction as to 

 indicate that the filament had suddenly become relatively 

 more electropositive to that extent. 



There seems no reason to doubt that most of this change 

 in the contact potential occurred at the heated filament. 

 Assuming that the whole change is at the filament, these 

 observations furnish the material for an approximate test of 

 equation (1). The test is only approximate in any event 

 because it was not possible to ascertain the temperature of 

 the filament with accuracy. When its inverse resistance 

 was 0*521 ohm -1 , corresponding to the left-hand curve in 

 fig. 1, its temperature was estimated with a Cambridge & 

 Paul Optical Pyrometer at 1583° K. Another estimate 

 can be obtained if we assume that it w^s giving the pure 

 tungsten emission at this stage. Using K. K. Smith's 

 tungsten data we calculate T = 1450? K; and using Langmuir's 

 T=1507° K for this limit, the average of these three esti- 

 mates is 1480° K. 



The emission for a resistance of ("521) -1 after the rise was 

 too large for us to measure with our instruments, but was 

 obtained by extrapolation. It was found that the currents 

 at various values of the inverse resistance were as follows : — 



i 2 (l =2-06 xlO- 10 amp.) 10-2 21-0 45 91"5 165 318 



log l0 f 2 1-01 1-323 1-653 1'962 2-218 2-503 



R'Vhm" 1 ) -774 '764 -740 -726 -714 -698 



The relation between the logarithm of this current and 

 ll _1 is found to be linear, and from the plot it appears that 

 the value of log 10 i 2 for 1/B, = *521 is 5*96. The value of 

 logical f° r 1/R = '521 was 2*62. Substituting the known 

 values of k and e and the experimental value of T, viz. 

 1480° K, and inserting the factors for volts and natural 

 logarithms, it appears that these data give V = - 96 volt as 

 compared with the measured value 0'71 volt. 



This is probably as substantial an agreement as could be 

 expected, considering the uncertainty of some of the data. 

 We are engaged in further experiments which it is believed 

 will eliminate these elements of uncertainty. The difficulties, 

 however, are not inconsiderable, and it is clear that some 

 time must elapse before further.results of value are obtained. 



A reference to these experiments was made by one of .us 

 in his Presidential Address to Section A of the British 

 Association at the Edinburgh meeting, 9th September 1921. 



