252 
PHYSICS: H. E. FARNSWORTH 
Proc. N. a. S. 
target out to the side of the tube where it could be heated to bright red 
heat by high-frequency induction currents. 
The ratio of the secondary to the primary electron current was studied, 
especially as affected by: (1) prolonged heating of the bombarded surface 
at bright red heat; (2) condition of the surface; (3) natiure of the gas to 
which it had been exposed. This ratio, for low primary velocities (less 
than 260 volts), decreased with prolonged heating, and the general shape 
of the secondary electron curve was changed. A Hmiting form of this 
curve was obtained, see figure 1, which was independent of whether the 
apparatus had previously been exposed to air or hydrogen; so it seems 
QQ \ \ 1 1 1 1 \ \ 1 ! \ \ \ \ \ \ \ \ \ 1 
0 2 4 6 6 10 12. 1^ 16 18 ZO ZZ 24 26 28 30 32 34 36 38 40 
Primary \je\oc\\\j in vo/fs 
FIG. 1 
safe to conclude that the final curve is one which represents the true 
characteristics of nickel. The number of secondary electrons was found 
to be independent of the roughness of the surface, but covering the surface 
with fine nickel gauze considerably reduced this secondary electron current. 
Secondary electrons were found when the primary velocity was as low as 
0.2 volt. In the limiting curve, see figure 1, the ratio of the secondary to 
the primary current increased rapidly with increase of the primary velocity 
to a value of 0.15 at about 4 volts, then remained constant to about 9 
volts, where it again increased, reaching a value of unity for a velocity of 
impact corresponding to 260 volts. 
