512 
MR. 0. W. RTCITARDSON OX THE ELECTRICAL CONDUCTIVITY 
does not become saturated till about 160 volts. It will be seen that this curve is 
very similar to the one given by Professor McClelland for the same pressure 
(•1 millim.). The similarity of the two leads to the conclusion that the form of the 
current E.M.F. curve is largely independent of the amount of ionisation produced Ijy 
the wire. In many of the experiments on the variation of the current with the 
temperature the pressure was considerably less than ’1 millim., but we should expect 
that a voltage which would saturate the current at a given pressure would saturate it 
at any lower pressure. At any rate, the experiments to be descriljed later show that 
this is true for the negative ionisation produced by hot carbon. Another set of 
experiments on platinum showed that at ’008 millim. the current was saturated by 
less than 80 volts. 
§ 5. The Relation between the Saturation Current and the Teni'perature of the 
Wire. 
The temperature of the wire was obtained from its resistance, and in order to 
determine this the apparatus indicated in hg. 2 was employed in the manner already 
described. During each observation it was found that the temperature of the wire, 
which was run at constant voltage, fell slightly, owing to the gas given off from the 
walls of the tube and elsewhere. A reading for the resistance was therefore taken 
immediately before and after the reading for the current, and the mean of the two 
resistances was taken to be that which corresponded to the current reading. The 
wire Avas heated for a long time and the tube constantly pumped out previous to 
