300 Mr McClelland, On the Action of Incandescent Metals 



pressure of the gas is atmospheric 1 , but after a certain potential 

 difference between the terminals has been reached the current 

 increases rapidly for further increases of potential. The curve 

 shows that the current at first increases proportionately with the 

 e.m.f., then shows signs of reaching a maximum and afterwards 

 increases rapidly. 



This second rapid increase of current can easily be explained 

 if we assume that the negative ions travelling from the negatively 

 charged wire to the cylinder C have the power of producing ion- 

 isation when they collide with molecules, provided the velocity of 

 the ion is sufficiently great. In the case we are dealing with the 

 e.m.f. has been sufficient to produce such a velocity in the air at a 

 pressure of § mm. When the potential difference is sufficiently 

 great to produce the required velocity the secondary ionisation 

 begins and then we have the rapid increase of current. At a 

 pressure of § mm. the negative ion has not the large mass 

 travelling with it that it has at atmospheric pressure and again 

 it is moving in a rare medium, so that it acquires a high velocity. 

 Prof. Townsend 2 has used the above theory to explain similar 

 results obtained with the ionisation produced by Rontgen radia- 

 tion in a gas at low pressure. The numbers given above refer to 

 an experiment in air at a pressure of § mm. We have selected 

 the results at that pressure because the secondary ionisation is 

 then more marked. At higher pressures the ions do not so easily 

 attain the necessary velocity, and at very much lower pressures 

 the secondary ionisation is smaller compared with the initial 

 ionisation produced direct by the wire. 



If we use a smaller current to heat the wire the secondary 

 ionisation is even more apparent than in the curve given above. 

 The negative ions have then a greater velocity, as we have seen 

 above. 



The following numbers refer to such an experiment, the wire 

 being at a lower temperature and a much smaller capacity joined 

 up to the electrometer. 



Potential of the 



Current in arbitrary 



wire in volts 



units 



40 



12 



80 



25 



120 



45 



160 



91 



200 



145 



280 



430 



1 Proceedings Cambridge Phil. Soc. Vol. x. Pt. iv. 



2 Phil. Mag. Feb. 1901. 



