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X. On the Resistance and Electromotive Force* of the Electric Arc. 



BIJ W. DUDDELL, Wh.Sc. 

 Communicated by Professor W. E. AYRTON, F.R.S. 



Received and Eead June 20, 1901, Received in revised foi-m October 2, 1903. 



[PLATE 2.] 



SINCE DAVY'S discovery of the electric arc, a century ago, down to the present time, 

 the nature of the physical processes going on in it, and the mechanism hy which it 

 conducts electricity, have heen the subject of almost uninterrupted discussion and 

 experiment. In order to explain the fact that the equation connecting P.U. current 

 and length appears to contain a large practically constant term, experimenters have 

 assumed that the arc possesses resistance and E.M.F., though which of the two is the 

 more important in obstructing the flow r of the current, or whether both must lie 

 considered, has been, and is still, a matter of controversy, the settlement of which, it 

 is hoped, will be furthered by the experimental results described in this communi- 

 cation. 



A priori it is highly probable that the resistance and E.M.F.'s of the electric arc, 

 if they exist, will be functions of the current ; it is therefore necessary to first 

 consider the definitions of these quantities, as it will largely depend on the definitions 

 adopted whether the arc can be said to possess a resistance, an E.M.F., or neither. 

 The ordinary text-book definitions of resistance and E.M.F. generally start with the 

 assumption that they are constant quantities independent of the current flowing, and 

 their possible variation is generally developed as a secondary effect due to the current 

 altering the state or nature of the body or apparatus considered, these alterations in 

 the state being the primary cause in the change in resistance and E.M.F. observed. 



There is much experimental evidence to support the view that when a steady 

 current, A, flows through any conducting apparatus, the potential difference, V, 

 between its terminals can be written V = E + RA when E and R, the E.M.F. and 

 the resistance, only depend on the nature, state and movement of the apparatus, and 

 are not directly functions of the current A or the potential V ; so that the equation 

 connecting the P.D. and current for any apparatus under perfectly constant conditions 



VOL. coin. A 368. 2 R 9.8.04 



