Titanium, and Vanadium by Thermelectronic Currents. 265 



the formation of the red fringe. Sir Ernest has further 

 been good enoll orh to calculate the strength of the magnetic 

 field for the particular case under consideration. Thus, with 

 a heating current of about 200 amperes the intensity of the 

 magnetic field at a point distant 0*5 mm. from the middle 

 of the plate is about 120 C.G.S. units, at 5 mm. from the 

 plate it is still 63 units? Now, with a heating current of this 

 strength the temperature of the plate is nearly 2700° C, and 

 under these conditions the red fringe is ordinarily not per- 

 ceptible to eye observations. Since, however, there is good 

 reason for presuming that the conductivity of the vapours in 

 the neighbourhood of a plate at this temperature must be 

 fairly high, w r e can only account for the absence of the red 

 fringe by supposing that the thermelectronic current is being 

 held in check by the magnetic field set up by the heating- 

 current. This view seems to receive strong support by the 

 fact already recorded in § 5 — namely, that the application 

 of a magnetic field of only about 125 C.G.S. units acting upon 

 the thermelectronic current in a downward sense, i e. in a 

 direction opposed to that of the magnetic force due to the 

 heating current, suffices to bring the red fringe into view. 

 Further, as will be shown in the next paragraph, when the 

 spreading downwards of the thermelectronic current is helped 

 by an additional magnetic field of sufficient strength, the red 

 fringe emission no longer stops abruptly, but dies out only 

 gradually. On the other hand, if in the case of the fully 

 developed red fringe, at a plate temperature of about 

 3000° C, an additional magnetic field be applied acting in 

 the same sense as that due to the heating current, the red 

 fringe is forced up against the plate, appearing as it were 

 in a state of compression, and its spectrum emission is now 

 seen to stop quite abruptly at the lower edge. These facts 

 seem to me to provide the basis for a satisfactory explanation 

 of the sharp outline presented by the red fringe. Supposing 

 there were no magnetic field set up by the heating current, 

 then the thermelectronic currents would be enabled to diffuse 

 freely through all those parts of the vapours which possess 

 the necessary degree of ionization. Now, since the latter is 

 primarily controlled by the plate temperature, the conduc- 

 tivity of the vapours would diminish only slowly on passing 

 dowmward from the plate. Consequently, the intensity of 

 the thermelectronic current would likewise fall off only 

 slowly on passing through regions of gradually decreasing 

 temperatures, and, as a matter of course, the red fringe 

 would not present a uniform band with a sharp outline, but 

 would fade away by degrees on passing downward from the 

 plate — a result which would be quite contrary to the observed 



