ARC IN AIR BETWEEN CARBON ELECTRODES 33 



in phase with such carbons it was necessary to have 

 frequencies of 100,000 or more per second. For cored 

 carbons 26,000 alternations per second were needed. 



When the alternating current was in phase with its 

 E.M.F. the ratio between the E.M.F. and the current was 

 taken as the "true resistance" of the arc, and the counter 

 E.M.F. was found from the formula E' = E RI. The 

 value of E' as found by him varied from 11.2 to 18.5 volts 

 with different currents and different carbons. 



" Forward E.M.F." He also endeavored to find the 

 part of the arc where this counter E.M.F was situated. 

 For this purpose he inserted an exploring pencil in the 

 arc and found the potential difference between this and 

 the anode, both for the direct and for the superimposed 

 alternating current. From this data he computed the 

 counter E.M.F. near the anode, using the same method 

 as that given above. This he found to be approximately 

 17 volts. A similar set of readings gave a "forward 

 E.M.F." near the cathode of 6 volts, the two accounting 

 for the ii volts which were found in this particular case 

 for the whole arc. 



This again is a use of words which in one sense is entirely 

 correct, and yet is entirely misleading. Duddell un- 

 doubtedly found the value of E RI near the cathode to 

 be a negative quantity, and since he had called this a 

 " counter E.M.F." when it was positive, it is not surprising 

 that it should be called a "forward E.M.F." when it was 

 negative. Yet to do so gives one the impression that there 

 is an E.M.F. at the cathode of the order of 6 volts, which 

 changes some other form of energy into electrical energy. 

 In reality there is no reason whatever to suppose that 

 there is any such E.M.F. at that point. All that Duddell's 



