IMPARTED TO A VACUUM BY HOT CONDUCTORS. 
537 
electromotive force for voltages in the wire electrode between 0 and + 40. The 
pressure was about 5 millims. The sudden increase in the current between 4 and 
8 volts was obtained every time and did not seem to 
be due to experimental error. 
The current E.M.F. curve from 40 to 240 volts is 
very similar to that in fig. 18, except that it approxi¬ 
mates very closely to a straight line between 160 and 
240 volts. It is given in fig. 19. 
When the voltage was increased above 240 it was 
found that the current rose rapidly to several thousand 
times its previous value. The increased current was 
quite steady at 320 volts, but at 280 volts it seemed 
to be in a very unstable state, since all kinds of 
intermediate readings could be obtained. Above 320 
volts the current increased in a linear manner with 
the voltage. The experimental numbers are given below. 
Volts on wire .... 
200 
240 
280 
280 
320 
360 
400 
Currents. 
Amptres x 10~'^ . . . 
3-6 
5-7 
93-6 
15.5-4 
7750 
14350 
20250 
These numbers seem to indicate that with potentials greater than 240 volts an 
ordinary vacuum discharge took place at some point or points in the tube ; in the 
following experiments care was therefore taken never to use potentials greater than 
80 volts 
§ 4. Relation hetiveen the- Current wider a given Voltage and the Temperature. 
In the case of sodium, owing to the fact that the current could not be saturated, 
its value under a given electromotive force was measured at different temperatures. 
This comes to practically the same thing as measuring the saturation current, since 
we should expect, ceteris paribus, the current v/ith a given electromotive force always 
to be proportional to the number of ions liberated at the metal surface. In order to 
be sure of not getting a discharge, a potential of about 80 volts between the wire 
A^B^, and the cylinder was always employed. The following table represents a series 
of observations of current and temperature ranging from 217° C. to 427° C. It will 
be seen that the corresponding range of current is from 10~° to 10“^ ampere; in 
other words, raising the temperature of the metal from 217° to 427° increases the 
current to ten million times its original value. The currents below 10“^ ampere 
were measured with the electrometer. In this series of experiments very low values 
of the currents were not measured; in a later series the current was taken nearly 
VOL. CCI.—A. 3 z 
