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



(Nos. 2 and 3) are peculiar, although similar observations- 

 have been recorded before by one of the writers *. The- 

 current is greater in the neighbourhood of 2-4 volts than it 

 is at somewhat higher potentials, indicating that one of the 

 effects of increasing the applied potential is to diminish 

 the number of systems giving rise to the emission. Curves 

 showing this maximum current at a low potential were 

 obtained after the wire had been heated at 379° C. for 

 50 hours, and then at a temperature of 600° C. for 2 hours 

 just before the observations were made. On the other hand, 

 some curves taken at intermediate intervals showed a con- 

 tinuous increase of current with increasing potential 

 difference. This shows that the maximum is not deter- 

 mined solely by some peculiarity in the initial condition of 

 the wire. We have not discovered what the determining 

 factor is. 



Fig. 3 shows that the proportional rate of decay of the 

 current at 400 volts is greater than that at moderate 

 potentials, for example, 200 volts, although this effect is 

 much less marked than in fig. 1. Thus, comparing curves 1 

 and 6, the ratio of the ordinates at 400 volts is about 5 to 1, 

 whereas at 280 volts it is only a little over 3 to 1. After 

 continued heating the results show a tendency to become 

 more irregular, but the main features exhibited in curve 6 

 were obtained too frequently to be likely to be due to obser- 

 vational errors. These features are (1) a steady rise to a 

 flat region between 80 and 160 volts, (2) a further rise from 

 160 to 280 volts, followed by (3) a comparatively flat region 

 between 320 and 400 volts. This type of curve seems to 

 point to the existence either of two different types of ions or 

 of two different processes involved in the emission. These 

 particular indications of duplicity were not noticeable in the 

 first apparatus. A later stage obtained after further heating 

 is shown in fig. 4, curve 2. 



We shall now consider a number of ways in which a tube 

 giving curves like those shown in fig. 3, curve 6, may be 

 made to exhibit curves like fig. 3, curve 1, again. This i& 

 found to happen if air at atmospheric pressure is admitted 

 into the tube, and then pumped out again before the obser- 

 vations are taken, and also if the wire is heated for a time 

 at a somewhat higher temperature than that at which the 

 previous and subsequent tests are carried out. The general 

 nature of these effects is the same in all cas^s, so that it will 

 be sufficient to consider one instance in detail. The wire in 

 the brass tube after continued heating gave the curve marked 



* O. W. Kichardson, Phil. Trans. A. vol. ccvii. pp. 10-11 (1906). 



