174 



Dr. C. Sheard on the Positive 



that operative in the lower temperature range, and the second 

 that which predominates at higher temperatures. Curves I., 

 II., and III. of fig. 1 show two maxima. The first maximum 

 In all three curves is at 654° C. ; in Curve III., showing the 



Fier. 1. 



kOo° febo° txjo 720* IScT ltd" »/o° ?40°C 

 Tern Ke rat u re 



slow-decay values of the current, the second maximum 

 appears at 700° C. instead of 756° C. as indicated in Curves I. 

 and II. This shift is explicable in the light of what follows 

 later in this paper. 



Moreover, experiments performed in another connexion, 

 while investigating the emission at -f 200 volts, showed the 

 existence of a maximum at about 650° C, when the tem- 

 perature was changed by short intervals from 630° C. to 

 900° C. The magnitude of this maximum decreased on 

 repetitions of the current-temperature series and finally 

 practically disappeared. On allowing the wire to stand cold 

 for several days the increased ionization effects were again 

 obtained, the maximum occurring at the same low tempe- 

 rature point. On the other hand, the positive currents 



