Titanium, and Vanadium by Thermelectronic Currents. 271 



of the former, are used, one above the other, and the 

 heating current divided between them, it is easy to see that 

 all the necessary conditions for the formation of the red 

 fringe will be appreciably improved. First, the better 

 shielding of the vapours in the space between the two 

 plates from the uprising air, and their exposure to a 

 radiating surface of twice the former area, will considerably 

 facilitate their ionization, which will, already at lower plate 

 temperatures, attain a high degree. Consequently, therm- 

 electronic currents will be enabled to pass under the action 

 of feebler electric Melds. Secondly, the heating current 

 through each plate will be markedly less and consequently 

 give rise to feebler magnetic fields ; but since the magnetic 

 forces due to each plate act in opposite directions in the 

 enclosed space, the resultant magnetic field will almost 

 vanish, and therefore the ionization currents will be able to 

 diffuse through the vapours without restraint. Lastly, any 

 possible photo-electric action will have a very much greater 

 value with a two-plate arrangement, because the two radiating 

 surfaces face each other. Now, as will be shown in a sub- 

 sequent communication, these considerations are fully borne 

 out by the observed facts, for with two parallel plates 5 mm. 

 apart the red fringe is observed already at 2200° C. with an 



actino- electric field of only about 4 - and, further, in 



° J cm. ' 



evidence of the absence of any appreciable magnetic field 

 within the region bounded by the two plates, the red fringe 

 is not confined to the immediate vicinity of the plate surface, 

 but fills out evenly the whole of the enclosed space. Yet, 

 notwithstanding the quite notable improvement observed, as 

 compared with the single plate, the loss of heat through 

 radiation and convection, in the case of two parallel plates, is 

 still very considerable, because the sides remain unprotected. 

 Hence we should expect to gain a further improvement bv 

 having these open sides likewise closed, and we are thus led 

 to anticipate that the best arrangement, fulfilling the con- 

 ditions enumerated, will be provided by a tube. Let us then 

 examine, with the help of all known facts about the tube- 

 furnace, in how far the conditions underlying the establish- 

 ment of thermelectronic currents are satisfied in the case of 

 a tube :— 



1. An extremely high degree of ionization in tube-furnaces 

 was first shown to exist by Messrs. Harker and Kaye. 

 These results were entirely confirmed by my own 

 experiments. 



