ELECTROMOTIVE FORCES OF THE ELECTRIC ARC. 333 



or the tearing-off of particles from the electrodes. There appears to he very little 

 evidence in favour of these last two explanations. 



The statement sometimes made that, as it requires a certain amount ot work to 

 be done to convert the solid electrodes into the gaseous state, and as this work is 

 done by the current, therefore the current must be flowing against a back E.M.F., is 

 without sufficient foundation. It is true that, in the above case, a P.D. will exist 

 which opposes the flow of the current, but that this P.D. is a reversible phenomenon, 

 and therefore an E.M.F., is not necessarily the case. The volatilization of the 

 electrode is of course reversible, but it requires experimental evidence to prove that 

 it is accompanied by a corresponding reversible electric phenomenon, so that the 

 energy, supposed supplied electrically, to cause the volatilisation, tends to be returned, 

 on condensation, in the form of electric energy. There is no evidence that any 

 appreciable E.M.F. is produced by the tearing-off of the solid particles from the 

 electrodes, and HERXFELD'S experiment of attracting these particles out of the arc by 

 means of an electrostatic field, which he says did not affect the P.D. or current 

 through the arc, seems to indicate that the back E.M.F. of the arc cannot be due to 

 this cause. 



That more than a small part of the back E.M.F. is due to a polarisation, such as 

 occurs in an ordinary cell, is difficult to conceive. If such were the case, what is the 

 nature of the chemical compound produced, and what becomes of it ? It is certain 

 that practically the whole of the energy supplied to the arc is emitted again as light 

 and heat, and that there is no considerable portion stored up in the products produced 

 by the arc. If, therefore, any chemical change takes place at the positive electrode 

 accompanied by absorption of energy, this energy must be given out again in the arc 

 or flame, and the reverse chemical change take place. Some of the energy might be 

 given out at the negative electrode, and account for the forward E.M.F. observed 

 there. The nature of the substance in which the chemical change must take place, 

 which change reverses and so forms a cyclic process, is an almost insuperable 

 difficulty, since the rate at which the energy must be constantly absorbed and given 

 out again is considerable, and the materials present in which this chemical change 

 must take place are only carbon, its vapour, and the slight trace of impurities which 

 the author thinks essential to the existence of the arc. 



If the impurity be assumed to be a salt of potassium, there is the possibility that a 

 carbide of potassium might be formed, and that part of the P.D. might be due to the 

 arc forming a cell, having carbon and potassium carbide as its electrodes, and the 

 vapour column as the electrolyte, the products produced by the flow of the current 

 through the cell being conceived to be destroyed in the flame which surrounds the 

 arc proper. 



That a chemical combination between the carbon and surrounding gas is not the 

 cause of the back E.M.F., at any rate in the normal silent arcs considered here, is 

 evident from the fact that the arc is but little affected by the nature of the gas in 



