PRODUCED BY HOT IM.ATINTM IN I'HTKIMA'T QA8E8 



III. THE IONISATION IN OXYGEN. 

 3. Current and Electromotive Fo, 



In considering the results of experiments under this head it is important to 

 ivmrmKtT that owing to the thinness of the wires used most of the fall of potential 

 occurred near the surface of the wire. For this reason there was always a consider- 

 ahle electric intensity near the hot wire even when the potential dilll-rence between 

 the electrodes was quite small. As a sufficient approximation for the experiment-^ 

 described in the next three sections we may take the electric intensity at the surface 

 of the wire to be 40 V for the wires O'l mi Him. in diameter, and 20 V for the wires 

 0'2 millini. in diameter, where V is the applied potential difference in volts. 



Except within certain limited ranges of pressure and voltage the positive leak from 

 a hot wire in oxygen was found to be independent of the applied electromotive force. 

 A saturation current therefore always exists, and it follows that, except in certain 

 special cases, the number of positive ions produced per second by a hot platinum 

 surface does not depend on the external electric field. For instance, an experiment 

 at 700 millims. pressure and a temperature of 793 C. gave the following values 

 of the current with different E.M.F.'s, the experiments being made in the order 

 indicated : 



The values of the current may be regarded as constant within the prolmble 

 experimental error. 



It is evident that at pressures near atmospheric, saturation is attained with a P.D. 

 of less than 40 volts. As a matter of fact, in these cases, where all the ionisation is 

 of one sign, there is, of course, no recombination and, provided the whole of the hot 

 wire is at a positive potential great enough to overcome the tendency to diffuse back 

 into the wire, Till the ions produced reach the collecting electrode. A saturation 

 current is thus obtained with a very small voltage. This is shown by some 

 experiments made at a pressure of the same order of magnitude, 528 millims , as in 

 tin- last experiment, and at a temperature of 70G C. The fall of potential along tlie 

 filament, due to the heating current, was equal to 3'3 volts. The currents obtained 

 with tin- various assigned -IIK-II, \oltages on the filament are as follows 



