Electricity through rarefied (jases. 791 



Table I. 



C(-) 



C(+) 

 XlO 8 amp. 



E— 



C ( _) 



V 



XIOOO mm. 



X IG 8 amp. 



C( 



+)-C(-) 



46 



268 





•21 



28 



36 



280 





•15 



21 



31 



255 





•14 



18 



24 



245 





11 



15 



26 



295 





•10 



14 



IS 



235 





•083 



12 



15 



280 





•056 



9 



13 



275 





•050 



8 



12 



285 





■044 



6 



6 



2(50 





•024 



3 



3 



295 





•010 



1(?) 



1 



240 





•004 



— 



emits also a large number of negative ions when being used 

 as a cathode. This is probably due to the fact that the salt 

 usually does not cover the whole surface of the strip, so that 

 the negative ions are emitted by the platinum surface itself. 

 In the experiments described above the strip was used as an 

 anode, but compared with the third electrode r it was always 

 charged to a negative potential of 200 v., so that the third 

 electrode received also the negative emission from the strip. 

 This source of error is comparatively small and may be 

 eliminated by insulating the cathode P and measuring the 

 current through the third electrode, all other conditions 

 remaining the same. When the t j mperature of the strip 

 was not very high, this source of error could be neglected. 



An attempt was also made to check the results by control 

 experiments. For this purpose the heated strip was coated 

 with calcium oxide and used as a cathode, being raised to 

 — 1800 v. The third electrode was brought to a potential of 

 — 2000 v. and the plate C earthed. The stream of electrons 

 or negative ions emitted by the strip produced by collision 

 with the gaseous molecules positive ions which were directed 

 towards the third electrode charged to a higher negative 

 potential, and the rate of production of positive ions by the 

 electrons was measured in the same way as before. The 

 strip was kept at a comparatively low temperature (dull red) 

 in order to avoid the discharge setting in. Since no positive 

 ions could be expected to be set free by the electrons from 



