IMPACTED TO A VACUUM BV HOT CONDUCTORS. 
511 
Resistance of wire. 
Current from wire to cylinder 
in amperes. 
ohms 
3-20 
+ 1-38 X 10-12 
3-30 
+ 3-7 X 10-12 
3-48 
+ 3-7 X 10-12 
3-78 
+ 1-6 X 10-12 
4-00 
-7-2 X 10-12 
4-22 
-7-5 X 10-11 
4-42 
- 6-8 X 10-10 
4-56 
-2-5 xlO-o 
It will be seen from these experiments and those to be described later that the 
negative ionisation increases very rapidly with the temperature, and becomes 
enormous compared with the positive. The transition temperature for platinum at 
low pressures is about 900° C. 
§ 4. The Relation between the Current and the Applied Electromotive Force. 
As the ultimate object of these experiments was to measure the saturation current 
from the wire, it was thought advisable to investigate the relation between the 
current and the j3otential aj)plied. A large number of current-E.M.F. curves for liot 
platinum have been given by Professor McClelland.* As, however, my apparatus, 
though similar, was not quite the same as, and the currents employed were much 
greater than, m the case investigated by Professor McClelland, it was considered 
necessary to make new ex 2 )erinients on the subject. 
As tlie absolute value of the current was continually varying in the way previously 
described, the current was continually referred to its value with a given potential on 
the wire. This “ standard” potential was — 41 volts. The current with 41 volts on 
the wire was measured both before and after taking a reading with any assigned 
potential: the ratio of this reading to the mean of the readings with 41 volts was 
taken to be what the ratio of the current under the given voltage to the current at 
41 volts would have been if the state of the tube had remained constant. In this 
way the variability of the hot wire could be satisfactorily eliminated. 
In these experiments the value of the saturation current was about 3 X 10~® 
ampere, and was probably about ten thousand times as big as the current used by 
Professor McClelland. In making the observations readings of the current were 
taken for every 6 or 7 volts u|j to 80, and afterwards at intervals of 40 volts up to 
400. The numbers so obtained are jjlotted in the following curve (fig. 3), the value 
of the current with — 41 volts on the wire is fixed arbitrarily as unity. The voltages 
refer to the positive end of the wire. 
The current rises to aljout one-third its final value with ten volts on the wire but 
* ‘Cambr. Phil. Proc.,’ vol. 11, p. 296. 
