426 
PHYSICS: IV. WILSON 
THE LOSS IN ENERGY OF WEHNELT CATHODES BY ELECTRON 
EMISSION 
By W. Wilson 
RESEARCH LABORATORIES OF THE AMERICAN TELEPHONE AND TELEGRAPH COMPANY 
AND OF THE WESTERN ELECTRIC COMPANY 
Communicated by R. A. Milliken. May 9. 1917 
It was first shown by O. W. Richardson in 1903 that the thermionic 
current from a hot cathode is given by the equation i = ad^e~^^^ where 
d is the absolute temperature and a and b are constants. 
The constant b has the special significance that it is proportional to 
the work done by an electron in leaving the surface of the body in ques- 
tion. This work can be determined directly by measuring the difference 
in power required to maintain a body at a certain temperature when it 
is emitting electrons from when it is not. Richardson and Cooke aiid 
later Lester have obtained values for tungsten which are in very good 
agreement with the values of b obtained by Langmuir and K. K. Smith. 
On the other hand Wehnelt and Jentsch, Schneider, Wehnelt and 
Liebreich, and Richardson and Cooke have all found that for lime cov- 
ered cathodes either the effect is so small in comparison with other 
energy changes as to be completely masked or that there is no corre- 
spondence between the two quantities. 
Since these experiments suggest that the mechanism of thermionic 
emission from Wehnelt cathodes is different from that for pure metals 
further experiments were made by the author to determine whether 
consistent results could be obtained by using more stable cathodes. 
If W is the work done by an electron in leaving the surface of a hot 
body W = bRy where R is the gas constant for one molecule. 
If it is assumed that the work done by the electron is that done by 
moving through a double layer of strength (p we have W == an d </> = 
bR/e. 
The method of Richardson and Cooke was used for the direct deter- 
mination of 4>. The constant b was determined in the usual manner, 
thermionic current being measured with the cathode at different tempera- 
tures which were obtained by means of an optical pyrometer of the Hol- 
born and Kurlbaum type. 
The following are the results obtained: 
