2 Sir Oliver Lodge on the Mode of Conduction in Gases 



The negatively-charged projectiles carry only an insig- 

 nificant proportion of the whole current, though they 

 may assist in propelling uncharged atoms towards the 

 anode, where they can become positive ions. 

 (3) The third point essential to an explanation is that 

 circulation of material in the tube is necessary, so as to 

 maintain a constant supply of positively charged ions, 

 and to enable them to act as carriers. 

 This last statement needs expansion : — 

 However plentifully electrons are supplied to the cathode 

 by the source of current, they do not spontaneously escape 

 from the metal (except under some stimulus such as ultra- 

 violet light provides) unless positively charged ions are in 

 their neighbourhood, i. e. unless the tension 2 77 <r 2 rises to a 

 critical value. 



In an active vacuum-tube the continual presence of such 

 ions near the cathode depends on their having been able to 

 travel from the anode ; nevertheless they are not atoms of the 

 anode itself, but are gas atoms which have become positively 

 charged by contact with it, — each of them having presumably 

 given up an electron to the metal. 



It is at the surface of the anode therefore that the sepa- 

 ration of electricities really takes place, under stress of the 

 applied E.M.F. The ions then migrate to the cathode, and 

 extract from it an electron apiece, thereby becoming neutral 

 again. 



A constant diffusion of uncharged atoms towards the anode 

 and of positively charged ions away from it is therefore 

 essential to the passage of current. The cathode bombard- 

 ment roused by the presence of the advancing ions can be 

 so planned or directed as to assist or to oppose the necessary 

 circulation.. The projectiles may assist by mechanically pro- 

 pelling uncharged atoms towards the anode, or they mav 

 oppose by mechanically driving back charged ions which are 

 trying to advance away from it. 



In doing the latter it would seem as if they ought to 

 neutralize opposing charges and thus convey some current ; 

 but the evidence points to the supposition that such flying 

 neutralization is rare, — it either does not occur or is but a 

 small fraction of the whole. The impacts shake, but perhaps 

 do not electrify or discharge, the ions : most of the inter- 

 change of electricity seems to go on only at or close to the 

 metal surfaces. 



If now a cathode is mounted so as to fire down a narrow 

 tube along which the positive carriers must come, it acts like 

 a park of Maxim guns obstructing a troop of infantry ; and 



