336 MR. W. DUDDELL ON THE RESISTANCE AND 



temperature obtained by the oxy-coal-gas flame, and the temperature differences and 

 gradients which may exist in the arc are probably many times greater than those 

 obtained in any of the above experiments, for it must be remembered that the carbon 

 at the lower temperature must always be at a bright red heat, or else it does not 

 appear to make electrical contact with the flame. It does not therefore seem 

 improbable that the P.D.'s of 1 volt or 1'5 volts obtained by unequally heating the 

 two carbons may have the same origin as the 10-volts to 18-volts back E.M.F. found 

 in the arc, especially as they agree both in direction and in the effect of impurities on 

 them. On this assumption, the probable causes of the back E.M.F. of the arc reduce 

 themselves to two, viz. : 



(1.) A thermo-electric force at the junction of the carbon and vapour, causing the 

 major part of the observed back E.M.F. 



(2.) A combination of carbon with the impurity present. 



Whether the thermo-electric force at the junction of carbon and vapour be due 

 either to the Peltier effect, or to a high temperature gradient at the contact, i.e., a 

 Thomson effect, or both, it seems at the present time to afford the most satisfactory 

 explanation of the back E.M.F. of the arc and the P.D.'s observed when two carbon 

 electrodes are unequally heated. The main objection is the great difference of 

 thermo-electric power the components of the junction must have, which, if the 

 difference of temperature be assumed to be only 1000 C., amounts to about 15 to 20 

 times that between bismuth and selenium. 



In favour of the view that the back E.M.F. is mainly due to thermal causes may 

 be mentioned the unilateral behaviour of the alternating arcs between a metal ball 

 and a metal point,* between carbons and metals, t and the observation of CROSS and 

 SHEPARDJ on the effect of cooling the positive crater of the direct-current arc- 

 All these experiments indicate that there is some cause existing which enables the 

 current to flow much more easily up the temperature gradient than down it. 



A tentative explanation of the chief causes which oppose the flow of the current 

 through the arc may be given under four heads : 



(1.) The resistance of the true vapour column, which is probably an electrolytic 

 conductor, whose conductivity greatly depends on the traces of impurities, such as 

 potassium and soda, present. Pure carbon vapour, like pure water, has probably a 

 very high specific resistance. 



(2.) A high contact resistance between the electrodes and the vapour column, 

 which leads to a large generation of heat, and consequently high temperatures and 

 temperature gradients at these points. 



(3.) Possibly a small back E.M.F., due to the electrolytic cell formed by the 

 electrodes and the vapour column as electrolyte. 



* ARCHBOLD and TEEPLE, 'American Journal of Science,' 1891. 



t DUDDELL and MARCHANT, 'Journal of the Institution of Electrical Engineers,' 1899, vol. 28, p. 74. 



I 'American Academy of Sciences,' 1896, p. 227. 



