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



intensity o£ the heating current, should receive careful 

 attention. 



When considering the action of an alternating current 

 furnace it is necessary also to bear in mind, that the diffusion 

 of the thermelectronic current through the ionized vapours 

 in the interior might, to some extent, be influenced by the 

 skin effect. This would, under certain conditions, cause 

 the complete stoppage of electric currents near the axis 

 of the tube. But it is to be presumed that, with ,the low 

 periods generally employed for heavy alternating currents — 

 namely from 50 to 100 per second, the thermelectronic 

 currents will nearly reach the centre. Dr. King has made 

 some interesting experiments which directly bear on this 

 point *. Two graphite exploring rods were inserted into the 

 tube from opposite ends. The inner end of one rod was 

 placed in the hottest part of the tube, and that of the other 

 rod in the cooler region near the end. A direct current 

 ammeter, placed in the circuit connecting the two exploring 

 rods, registered a current of about 1*5 amperes at a furnace 

 temperature of about 2600° C. The rod in the hot part had, 

 of course, become a cathode and, together with the cooler 

 anode, acted as a rectifier for the alternating current, part of 

 which leaked across the space from the inner wall to the 

 rod in the centre. Thus there *is little doubt that in 

 Dr. King's furnace thermelectronic currents will be enabled 

 to pass very near to the axis of the tube. Dr. King has 

 further shown that the degree of ionization decreases rapidly 

 with increasing pressure ; it is therefore important to record 

 the fact that most of Dr. King's observations on furnace 

 spectra were made at low pressure, whereas all mine have 

 been carried out at atmospheric pressure. 



With regard to the line emission of iron which is caused 

 by thermo-chemical excitation, there is little doubt that its 

 state of development at a given temperature, in the alter- 

 nating current furnace, will be the same as that observed in 

 the continuous current one. But, with the former type of 

 furnace, especially when the tube is unprotected, the second 

 line emission — namely, that due to thermo-electrical exci- 

 tation, becomes perceptible already at the lower furnace 

 temperatures ; consequently, the iron spectrum which is 

 observed under these conditions will, already from the 

 outset, be the result of the superposition of two different 



* A. S. King, Astroph. Journ. vol. xxxviii. p. 831 (1913). 



