504 Positive Emission Currents from Hot Platinum, 



instance, the current, after continued heating with the wire 

 positively charged at 342° C, gave an almost constant current 

 which only increased from 0'6 div. per sec. at +120 volts 

 to 0*75 div. per sec. at +400 volts. The wire was then 

 heated for 30 minutes at 595° 0. and -400 volts. The 

 negative emission was measurable at this temperature. On 

 returning to 342° C. and +400 volts, the initial current was 

 found to be equal to 14 divisions per sec, or about 20 times 

 the previous value. This large current fell away fairly 

 rapidly with time at the lower temperature (342° C), as is 

 shown in the following numbers : — 



•Current in di vs. per sec. 14 10 8 51 3-4 26 1-8 125 1-05 l'OO 

 Time in minutes 5 10 25 35 45 60 75 85 100 



The negative currents which were used to stimulate the 

 tube in these experiments were quite small. They varied 

 a little, but were always of the order of 10 ~ 13 amp. The 

 relation between current and potential for the positive leak 

 in the stimulated tube appeared to depend a good deal on its 

 previous history. The curves obtained at first showed an 

 almost linear relation between current and potential from 

 40 to 400 volts, after allowing for the time factor. With 

 successive stimulations the curves became more concave to 

 the potential axis, until finally they were almost saturated 

 all the way from +40 to +400 volts. It appears therefore 

 that, with successive treatments, the tube gets into such a 

 state that the passage of a negative discharge from the wire 

 at a higher temperature produces an increase in the posi- 

 tive emission, subsequently measured at the original lower 

 temperature, which does not increase with the applied 

 potential at high potentials. This increase is still of the 

 order of magnitude, even at +400 volts, of that which is 

 observed in the earlier stages and dies away with time at a 

 rate which is of the same order of magnitude. 



The progressive increase of the currents, with increasing 

 potential difference, which has been recorded in the various 

 cases tested, might be due to a variety of causes. Recom- 

 bination, which is the most important factor preventing the 

 attainment of saturation in the usual cases of ionization of 

 gases at moderate pressures, does not occur here, as there is 

 no current, at the temperatures at which the positive emis- 

 sion was measured, when the wire is charged negatively. 

 Ionization of the surrounding gas by collisions would not be 

 expected to be a serious factor on account of the smallness 

 of the pressures. We are thus limited either to an effect of 



