Electromotive Forces of the Electric Arc. 



517 



The connection between the resistance r and the current A for the 

 cored arc, length 3 mm. between 11 mm., "Conradty Noris" carbons, 

 can be approximately expressed over the range 1*5 to 20 amperes by 



(r + 0-25) A = 29. 



For the solid arc, length 3 mm. between the same size and make of 

 carbons, and over the range 1*5 to 11 amperes, the relation is 



33-5 , 42 

 7 = ~A~ A? 



Effect of Varying the Arc Length. 



The direct current through the arc being kept constant, the change 

 in resistance and back E.M.F. due to change of arc length is examined. 

 It is found that both for solid and for cored arcs increasing the length 

 increases the resistance, the curves between resistance and length 

 being very similar to those between P.D. arc and length. This latter 

 curve is generally assumed to be a straight line for solid arcs, but 

 such was not the case over the wide range of length, 1 to 30 mm., used 

 for these experiments. 



Effect of Varying the Nature of the Electrodes. 



Both the resistance and the back E.M.F. are found to depend 

 greatly on the composition of the electrodes ; thus simply soaking a 

 pair of solid carbons in potassium carbonate, reduced the resistance of 

 the arc between them from 3 "81 to 2 '92 ohms, and increased its back 

 E.M.F. from 12 volts to 15 - 6 volts, Lhe arc length and direct current 

 being kept constant : similar results were produced by introducing other 

 impurities. The author is of the opinion that the resistance of an arc 

 between perfectly pure carbon electrodes would be very high, so high 

 that it might be impossible to maintain a true arc, and that traces 

 of impurities are essential to provide the carriers of the electric 

 charges in the vapour column. 



Seat of the Back E.M.F. 



In order to determine whether the back E.M.F. and resistance are 

 localised at the electrodes, or are distributed along the vapour column, 

 a search carbon was introduced into an arc 6 mm. long between 

 solid " Conradty Noris " carbons, 11 mm. diameter, current 9*91 amperes. 

 The impedance to the high frequency testing current, of that part of 

 the arc between the search carbon and each of the main carbons, was 

 measured for three different positions of the search carbon. From 

 these tests it is deduced that the resistance of the above arc, as a 

 whole, consists of three parts — a resistance at or near the contact of 

 the positive electrode and the vapour column of about 1*61 ohms ; the 



