IMPAETED TO A VACUUM BY HOT CONDUCTORS. 531 
The following table represents a series of observations taken with tlie thermocouple 
apparatus at somewhat higher temperatures. The potential on the filament was here 
= — 87 volts. 
The temperatures given by the thermocouple method are on the average about 
120 lower than those obtained from the resistance for the same current. The 
temperature registered by the couple would be lower than that of the more remote 
paits of the filament for several reasons, the chief one being the conduction of heat 
away locally by the leads of the thermocouple itself It is difiicult to say whether 
we sliould expect this difference to amount to 120° C. 
We aie now in a position to test whether the experimental results are in agreement 
with the theoretical formula for the saturation current, viz. :_ 
C/eS = n aJ 
V 2mTT 
usmg tlie notation employed before. If we take, as in the case of platinum, 
y —i logio^ and Xq = the above equation reduces, as before, to the 
straight line 
y = ^ 
1>qXq. 
The following curve (fig. 15) has been plotted in tliis manner from the numbers 
gis^en in the table on p. 525. The ordinates are values of log^^jC — 1 log^Q0. while the 
abscissae are values of X 10^. 
3 Y 2 
