498 Profs. 0. W. Richardson and C. Sheard : Variation of 



actuated by the main heating-current in the usual way. 

 The potentials were applied at the mid-point of a high 

 resistance which shunted the wire outside the tube : they 

 therefore represent the average potential of the hot wire. 

 We aimed to work at as low temperature and with as small 

 currents as possible so as to eliminate the time-decay of the 

 positive emission. The currents were measured with a deli- 

 cate electrometer. The platinum wire in the glass tube was 

 Johnson & Matthey's purest resistance wire; that used in 

 the other experiments was ordinary commercial platinum 

 and had a very low temperature coefficient of resistance 

 (•00165 per ° C). The pressure recorded on the McLeod 

 gauge was between the limits 0*00005 and 0*0002 in all the 

 experiments. 



In the first observations with the glass tube and platinum 

 plate-electrode the temperature was adjusted to 451*5 ±1 0, 5 C. 

 Successive curves showing the relation between the current 

 and applied voltage at different times are exhibited in fig. 1. 

 The first current-E.M.F. curve was not completed until about 

 2 J hours had elapsed from the commencement of the heating, 

 as some time was occupied in preliminary adjustments. It 

 is marked A in the figure. The curve is slightly convex 

 to the voltage axis, but is almost linear, in agreement with 

 the one referred to above published by one of the writers. 

 The approximately linear relation did not hold down to V = 0, 

 but the curves became concave to the voltage axis at low 

 voltages as shown in curve B, which is on a different scale. 

 All the potentials in fig. 1 are about 4 volts too high on 

 account of the measured potential not being that at the 

 middle of the hot wire. 



As the heating of the wire was continued the first effect 

 was to rotate the curve A about O towards the potential axis. 

 This corresponds to a decay of the currents in approximately 

 equal proportions at all potentials. With further heating the 

 curves developed a flat region in the middle, as is shown by 

 curve C, which was obtained after 11 hours heating. With 

 subsequent heating the increased current above 280 volts 

 continues to decrease faster than the currents at lower 

 voltages, until after about 30 hours it is scarcely noticeable. 

 The result of heating for 27^ hours is shown in curve D. 

 The observations both with rising and with falling potentials 

 are shown in the figure. The difference between them is not 

 very great in most instances, but when there is an appre- 

 ciable difference the currents with rising potentials, which 

 were measured first, are larger (see below). It is important 

 to remember that in these experiments the temperatures were 

 so low that several hours were necessary for the emission to 



