302 Mr McClelland, On the Action of Incandescent Metals 



Potential of 



Current in arbitrary 



wire in volts 



units 



8 



19 



18 



22 



40 



24 



120 



25 



240 



27 



280 



29 



320 



37 



360 



50 



The curve in Fig. 4 is plotted from these numbers. 





















































































































8 







































5 























































































































































100 



90 



80 



70 



60 



50 



40 



160 200 



Volts. 

 Fig. 4. 



320 



The pressure of the air during the above experiment was 

 | mm. and the temperature of the wire the same as in the experi- 

 ment illustrated by the curve in Fig. 2. The conductivity when 

 the wire is positive is very much less than when the wire is 

 negative, as shall be referred to later in the paper. The above 

 numbers are not to be compared as regards their absolute values 

 with the numbers in the previous experiment when the wire is 

 negative. 



The rapid increase of current shown in the curve in Fig. 4 

 after the e.m.f. has reached a certain value would seem to indicate 

 that the positive ions travelling out from the wire have produced 

 secondary ionisation. The e.m.f. for which the secondary ionisa- 

 tion begins is not very different in the two cases, which would 

 show that the masses in the two cases are not very different. All 

 investigations on ionisation produced in air at pressures of 1 mm. 

 and less, by Rontgen rays and ultra-violet light as well as in the 



