Vol,. 8, 1922 
PHYSICS: H. E. FARNSWORTH 
251 
providing we add the further conditions that the standard deviations in 
all of the primary causes are equal and that the effects of all of the primary 
causes are equal. These conditions appear at first very limited, and in a 
practical case it is necessary at least qualitatively to check the assumptions 
underlying equation (4), but in general it has been found that a significant 
deviation in r may be taken to indicate a variation in the effects of one or 
more groups of primary causes, upon one of the variables and not upon the 
other. 
A more complete account of this investigation giving experimental data, 
will be published elsewhere. 
^ Pearson, K. "Tables for Statisticians and Biometricians" pp. (26-28) and Phil. 
Mag., Ser. 5, 1, 1900. 
2 Edgeworth, F. Y., Camb. Phil. Trans., 20, 1904 (36-65) and (113-141). 
ELECTRONIC BOMBARDMENT OF NICKEL 
By H. E. Farns worth 
Department of Physics, The University of Wisconsin 
Communicated June 13, 1922 
Studies of the secondary (emergent) electrons produced by electronic 
bombardment of metal surfaces have previously been made by various 
experimenters.^ However, the fact that most of the results were obtained 
previous to the development of modern high vacuum technique combined 
with the failure of the small amount of recent work to agree with these 
older results, leaves the important questions of this problem still un- 
answered. It is well known that the number of secondary electrons de- 
pends upon the velocity of the primary (incident) beam, that more elec- 
trons leave the surface than strike it if the primary velocity is great enough, 
but the following characteristics of the various metals are still not definitely 
determined: (1) the magnitude of the secondary electron current as a 
function of the primary velocity; (2) the velocity distribution of the 
secondary electrons for any given primary velocity. 
The present investigation is an attempt to obtain these characteristics 
for nickel. The type of apparatus employed was such that a primary 
electron stream, of uniform velocity and cross-section, could be made either 
to impinge on a nickel target or to pass into an absorbing Faraday cylinder. 
This was accomplished by moving the target, by means of a magnetic 
control, over a hole in the Faraday cylinder into the path of the primary 
stream. The secondary electrons which left the target were received by 
a conducting cylinder. This arrangement made it possible to move the 
