ELECTROMOTIVE FORCES OF THE ELECTRIC ARC. 331 



electrodes than elsewhere, the above resistances are probably located at the contact 

 between the electrodes and the vapour column. 



The resistance of the arc as a whole may therefore be considered to consist of three 

 parts, the resistance of the vapour column and the resistances of the two contacts 

 between it and each of the electrodes. 



The mean value of the back E.M.F. of the arc lamp, with the search carbon in 

 place in the arc, i.e., 12'3 volts, is practically identical with its value of 127 volts, 

 obtained without any search carbon in the arc. The back E.M.F. of that part of the 

 arc between the search carbon and the positive electrode had, for the three different 

 positions of the search carbons, the values IG'G, 17 '9 and 15 '6 volts respectively, 

 which may be considered as indicating that this back E.M.F. is independent of the 

 position of the search carbon to within the limits of accuracy of the present experi- 

 ments. The back E.M.F. between the search carbon and the negative electrode 

 calculated in the same way, i.e., by subtracting the product of resistance by direct 

 current, or ohmic drop in this part of the arc, from the observed direct P.D. between 

 the search carbon and the negative electrode, is a negative quantity, so that there 

 exists a forward E.M.F. which helps the flow of the direct current. The value of 

 this forward E.M.F. was 6'5, G'4 and 5'3 volts respectively, again practically inde- 

 pendent of the position of the search carbon. 



These experiments show that there is in this arc at or near the positive electrode 

 a back E.M.F. having a mean value of 107 volts, and at or near the negative 

 electrode -A forward E.M.F. of G'l volts. The fact that the algebraic sum of these 

 two voltages is not equal to the observed mean back E.M.F. 12'3 volts of the arc, as 

 a whole, is probably caused by the distortion of the lines of flow of the current 

 through the arc, produced by the introduction of the search carbon, which renders 

 the sum of the resistances measured between each of the main electrodes and the 

 search carbon greater than their total as measured between the main electrodes. 



The back E.M.F. of the arc as a whole consists, therefore, of two E.M.F.'s located 

 at or near the electrodes, the larger E. M.F. situated at or near the positive electrode 

 opposing the flow of the current, the smaller E.M.F. situated at or near the negative 

 electrode helping the flow of the current. 



The approximate values of the resistance E.M.F. of each part of the arc, and the 

 drop of volts due to each of these causes, corrections for the self-induction and 

 resistance of the holders and carbons being omitted as before, are given in Table VI. 

 The P.D.'s in this table multiplied by the direct current of 9'91 amperes give the 

 power supplied to each part of the arc. 



2 u 2 



