IMPARTED TO A VACUUM BY HOT CONDUCTORS. 513 
niaking the observations, in order to reduce the evolution of gas as far as possible. 
When the tubes were heated at first large cpiantities of gas came off, but after a time 
further heating and jmmping did not seem to eftect much reduction in the rate of 
evolution of gas. Despite constant pumping the pressure always rose slightly in the 
McLeod gauge; the increase was, of course, more marked the higher the temperature 
of the wire. 
To obtain the temperature from the resistance measurements use was made of the 
determinations of the melting points of potassium and sodium sulphates of Messrs. 
Heycock and Neville.'^”^ The wire was set up in air and its resistance determined, 
first at the ordinary temperature and afterwards when the smallest possible grain of 
potassium sulphate placed on it just melted. In this way the resistance for two 
tempeiatnies differing by about 1000° was obtained, and the temjierature corresponding 
to any other resistance reading could be got by interpolation from the ciiiwes given by 
Piofessor Callendae. f To test the method the melting point of sodium sulphate 
was determined and no determination was more than 20° from the true value 
(883 G.). dhis agreement was held to be quite good enough for the purpose. The 
tempeiatine as found from the resistance in this way is the average temperature of 
the wire whereas what is required is the temperature at the surface. A calculation 
showed, however, that the temperature at the centre of the hot wire only differed 
from that at the circumference by 4° C. , a quantity which is negligible comj^ared with 
the experimental error. 
In the experiments on platinum a potential of 120 volts was maintained on the 
wile; this was more than enough to saturate the current at practically all the 
piessuies which occurred, it was found afterwards that at pressures greater than 
09 milhm. the current was not saturated by this potential but the deviation from 
the saturation value due to this cause is smaller for the observations taken than the 
error due to unavoidable irregularities. 
The values of the pressures are given in the tables foi‘ comparison, Two numbers 
are inserted in each case in the resistance column, these are the resistances as deter¬ 
mined immediately before and after the value of the saturation current was read. 
The difference between the two numbers is a measure of the rate at which the 
tempeiatine of the wire was changing and, therefore, of the rate at which the pressure 
of the gas in the tube was increasing. 
The following table gives the results which were obtained at temperatures below 
1450° C. 
* ‘Jour. Chem. Soc.,’ vol. 67, p. 160. 
t ‘ Phil. Mag.,’ vol. 48, p. .519. 
3 u 
VOL. CCI.—A, 
