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



2. In a tube-furnace which is well protected from loss 



of heat by means of a thick layer of carborundum 

 powder, the heating current necessary to raise the 

 tube to a certain temperature will naturally be less 

 than if the tube were not so protected ; for there are 

 practically no heat losses to make good as compared 

 with the single plate or even the two-plate arrange- 

 ment. Consequently, with such a tube-furnace, the 

 drop of potential along the tube will be relatively 

 small and, in my particular case, it was only about 



1*05 T L for a temperature of 2600° C, as against 



cm. r ' to 



5*5^— s in the case of the sing-le plate. Hence, the 



cm. © i > 



acting electric field in a tube-furnace protected 

 against heat losses in the manner stated is rela- 

 tively small and may even not suffice, especially at 

 the lower temperatures, to produce all those spectro- 

 scopic effects which were observed with the single 

 plate. 



3. The magnetic field inside a tubular conductor is zero *. 



Thus, if the conductivity of the vapour has the 

 necessary value, thermelectronic currents will pass 

 absolutely unimpeded by magnetic forces and be con- 

 trolled solely by the degree of ionization prevailing 

 in the interior. 



4. Photo-electric actions should be very great in this case 



since every point is acted upon from all round the 

 circular wall. 



Thus we see that, with the possible exception of con- 

 dition 2, there is a great likelihood that the electric actions, 

 which have been shown to underlie the luminous phenomena 

 constituting the red fringe, should also prevail in the interior 

 of a tube-furnace. 



Let us then examine more closely in how far the spectro- 

 scopic results obtained with the tube resistance furnace 

 agree with my observations on the spectrum of the red 

 fringe. Since the magnetic field inside the tube-furnace is 

 zero the red fringe, or its equivalent, should fill out the whole 

 space, as in the case of the two parallel plates. Tt will be 

 remembered that I found the light given out by the interior 

 of the tube-furnace, up to a temperature of irom 2400 to 

 '2500° 0., to be of purple tint f- This., of course, suggested 



* Sir J. J. Thomson, Elements Mathem. Tlieoiy of El. & Masrn., 

 3rd ed. p. 333. 



f Hemsalech, Phil. Mag. vol. xxxvi. p. 214 (1918). 



