270 Mr. Gr. A. Hemsalech : Excitation of Spectra of Carbon, 



case of a tube-furnace. These conditions may be summarized 

 as follows : — 



1. High degree of ionization of the gases and vapours in 



the neighbourhood of the hot graphite; 



2. An electric field of sufficient strength to drive an 



electric current through the ionized vapours and 

 gases ; 



3. A minimum magnetic field due to the heating current, 



so as not to restrict the spreading out of the therm- 

 electronic car rents ; 



and to these may perhaps be added, 



4. Maximum photo-electric actions favouring the emission 



of electrons. 



Before discussing the particular case of the tube-furnace 

 it will be useful to recall the difficulties which were en- 

 countered in the endeavour to realize the necessary con- 

 ditions with the simple plate. It was shown in this case 

 that the loss of heat by radiation and convection was 

 very considerable, and therefore, in order to maintain the 

 plate at a high temperature, very heavy electric currents 

 had to be forced through it. Further, owing to the upward 

 rush of cool air from below the plate, the ionization of the 

 vapours to the desired degree was always difficult to effect, 

 and its accomplishment necessitated a very much higher 

 plate temperature than would have sufficed in the absence 

 of air draughts. As we have seen, the minimum plate 

 temperature at which the spectroscopic effects of the therm- 

 electronic current begin to manifest themselves is about 

 2700° C. The heating current required to attain this 

 temperature gives rise to a magnetic field of such magni- 

 tude as to seriously check the spreading out of the therm- 

 electronic current. Also, since photo-electric actions in the 

 case of a plane surface can only take place between imme- 

 diately adjacent points, the emission of electrons from this 

 cause cannot be very great. Hence we find that in the 

 case of an electrically heated plate, some of the es-ential 

 factors in the formation of the thermelectronic current are 

 either unfavourable or even opposed to it, and the desired 

 effect was only obtained by passing a heating current of high 

 density through the plate, entailing the establishment of an 



electric field of up to 8 — — . Now, if instead of a single 



L cm. ° 



plate, two parallel plates, each of only about half the thickness 



