Resistance of the Electric Arc. 351 



current is proportional to the length of the tube plus a con- 

 stant. The curve we have obtained is also strikingly like that 

 obtained by Drs. W. De La Rue and Hugo Miiller for the 

 connexion between the electromotive force and the distance 

 across which it would send a spark*. These gentlemen also 

 made experiments on the electric arc with their large battery ; 

 but we do not find recorded any results with carbon points. 

 On page 185 of the reprint from the ' Philosophical Transac- 

 tions ' of the account of their researches, the result of an 

 experiment in air between two brass points is given ; but 

 according to that, when the arc was half an inch in length the 

 difference of potentials between the brass points was that of 

 657 of their cells, or about 700 volts. How far the very high 

 electromotive force found by Drs. W. De La Rue and Hugo 

 Miiller, to be necessary in this case, arose from a combination 

 of the material employed for the electrodes and the smallness 

 of the diameter of the brass electrodes, or whether the law that 

 " the electromotive force necessary to maintain an arc depends 

 mainly on the length of the arc and hardly at all on the 

 strength of the current " fails when the current is below a cer- 

 tain small limit, we are unable to say ; but of course both the 

 diameter of the brass electrodes they employed and the 

 strength of the current that was passing (0*025 ampere) 

 in the arc was very much less than that used in any 

 ordinary electric light, and to which the experiments of Mr. 

 Schwendler and ourselves especially refer. It is very pro- 

 bable that the difference in the material of the electrodes has 

 mainly to do with the difference between their results and 

 ours; and we think it very probable that with very soft carbons 

 an arc of a given length could be maintained with a much less 

 difference of potentials than that found by us, since it would 

 be more easy for a shower of carbon particles to be main- 

 tained between the ends of the carbons. 



We have used as the title of this short communication the 

 Resistance of the Electric Arc ; but we are perfectly aware of 

 the objections to this expression. How far the opposition to 

 the passage of the currents in an electric arc is due to pure 

 resistance, and how far to an opposing electromotive force, is 

 up to the present time by no means certain. That there is 

 some opposing electromotive force, seeing that mechanical 

 disintegration of the carbon and transporting of its particles 

 occurs, is, as was pointed out some years ago by Edlund, 

 almost certain ; but seeing that this opposing electromotive 

 force ceases to exist with the extinction of the arc, and 



* Page 32 of the Reprint of " Experimental Researches on the Electric 

 Discharge," Phil, Trans, part i. vol. 169. 



2C2 



