518 Resistance and Electromotive Forces of the Mectric Arc. 



resistance of the vapour column, about 2 '5 ohms ; and a resistance at 

 or near the contact between the vapour column and the negative 

 electrode of about 1*18 ohms. 



The back E.M.F. consists of two parts located at or near the 

 contact between the electrodes and the vapour column. That at the 

 positive electrode, about 1 7 volts, opposes the flow of the direct current 

 while that at the negative electrode, about 6 volts, helps the flow of the 

 direct current, i.e., is a forward E.M.F. 



Conclusion. 



The author considers that the new facts given in the paper assist in 

 formulating a consistent explanation of the resistance and back E.M.F. 

 of the arc. The values found for the resistance of the vapour column 

 and for the contacts between it and the electrodes offer no serious 

 difficulties. The greater part of the two E.M.F.'s are considered as 

 being most probably due to thermo-electric forces, and experiments 

 in support of this view are described, in which it was found 

 possible to obtain a P.D. of 0*6 volt by unequally heating two solid 

 carbon electrodes with a blow-pipe flame, the voltmeter indicating 

 that the hotter carbon was positive to the cooler. By using cored 

 carbons and adding potassium salts, this P.D. was increased to 1*5 volts. 

 It is pointed out that the differences of temperature existing in the 

 arc must be many times as great as those which it is possible to 

 produce with the blow pipe, as the cooler electrode must be red 

 hot, or else it does not seem to make contact with the surrounding 

 flame. 



On the Resistance of an Electrolyte. 



In measuring the resistance of an electrolyte by the Kohlrausch 

 method, it is often assumed that the errors clue to polarisation are 

 avoided if the frequency of the alternating or interrupted current used, 

 is as high as a few hundred periods per second. To investigate the 

 accuracy of this assumption the arc was replaced by a cell containing sul- 

 phuric acid, density 1*20 (temperature 20° C), as the electrolyte, and its 

 impedance and power factor tested exactly the same way as those of 

 the arc. It is found with this cell that it was not until the frequency 

 exceeded 10,000 periods per second that the electrolyte behaved as a 

 non-inductive resistance, and the errors due to the polarisation were 

 avoided. If the resistance of this cell were tested in the ordinary 

 way at a frequency of 100 periods per second, the value obtained would 

 be over twice its true resistance. It is concluded that unless other 

 methods are adopted to eliminate the effects of polarisation, it must not 

 be assumed that the use of alternating currents of ordinary frequencies of 

 a few hundred periods per second, eliminates the possibility of errors due to 

 polarisation. 



