from an Arc and from Incandescent Metals. 



253 



temperatures the rate of discharge of positive electricity is approxi- 

 mately the same as that of negative. 



(20) The conductivity produced by the incandescent wire is 

 such as is produced by the ionisation of a gas ; the current varies 

 with the e.m.f. in the usual way, and the gas loses all its con- 

 ductivity when the maximum current it can carry has passed 

 through it. The numbers below refer to an experiment in which 

 the rate of leak of the terminal B, Fig. 1, is measured for different 

 values of the E.M.F. when the conducting gas is forced past it at a 

 constant velocity. This experiment may be made with the 

 terminal charged either positively or negatively if the wire is at a 

 sufficiently high temperature, but with the wire at a dull red heat 

 the terminal will leak only if charged negatively. 



Volts. 



200 



120 



80 



40 



20 



9 



Current in arbitrary units. 



50 

 47 

 44 

 26 

 145 

 7 



Fig. 4 shows the curve plotted from these numbers, and we see 

 that a maximum value of the current is reached in the same way 

 as with the conductivity produced by Rontgen rays or by the arc. 



20 40 60 



100 120 140 160 180 200 

 VOLTS 



Fig. 4. 



The conductivity is therefore produced by the ionisation of the 

 VOL. X. PT. IV. 18 



