516 



Mr. W. Duddell. On the Resistance and 



the power factor increases rapidly at first, then more slowly becoming 

 asymptotic to + 1, and finally practically attains this value at 90,000 

 periods per second ; above this frequency the power factor is within 

 the limits of experimental error + 1 up to the highest frequency used, 

 viz., 120,000 periods per second. The impedance of the solid arc 

 increases with increase of frequency from 0'97 ohm at 250 periods per 

 second to 3*8 ohm at 90,000 periods per second, above which it remains 

 practically constant. The true resistance of the above arc 3 mm. long 

 between 11 mm. solid " Conradty Noris" carbons, and through which a 

 current of 9*91 amperes is flowing, is found to be 3 '81 ohms. 



The P.D. accounted for by ohmic drop is therefore 37*8 volts out 

 of an observed P.D. arc of 49'8 volts, so that there appears to be a real 

 back E.M.F. opposing the flow of the current, in this arc of 12 volts. 



With cored carbons the power factor at 250 periods per second is 

 + 0*67, and it increases until it is practically + 1 at 15,000 periods 

 per second, and remains unity within the limits of experimental error 

 up to the highest frequency tried of 50,000 periods per second, the 

 impedance becoming practically constant as with solid carbons. The 

 true resistance of the above arc 3 mm. long between 11 mm. cored " Conradty 

 Noris" coirbons, and through which a current of 10 amperes is flowing, is 

 found to be 2*54 ohms and the back E.M.F. 16'9 volts. 



The fact that the solid arc has, at low frequencies, a negative power 

 factor, indicates that the arc is supplying power to the alternator : 

 this is shown to be the case by means of a wattmeter. This is not, 

 of course, at variance with the principle of conservation of energy, 

 as the alternating energy given out by the arc is derived from the 

 direct current energy supplied to it. This fact that the solid arc is 

 capable of transforming, under suitable conditions, direct current into 

 alternating current is the basis of the " Musical Arc " recently shown 

 for the first time, at the Institution of Electrical Engineers. 



Effect of Varying the Direct Current. 



Having found that it is possible to measure the true resistance and 

 back E.M.F. of the arc, the effect of changing the direct current, the 

 arc having a constant length of 3 mm., is examined. 



The resistance of both the solid and the cored arcs is found to 

 increase with decrease of the current through the arc, apparently 

 tending to become infinite for current O. 



The back E.M.F. of the solid arc first decreases with increase of 

 current and then increases again, having a minimum value of 11*3 volts 

 at about 6 amperes. With cored carbons the back E.M.F. increases 

 with increase of current from 12*2 volts at 1 ampere to 18*5 volts at 

 20*8 amperes. The high P.D.'s required to maintain small current 

 arcs are shown to be due to the high resistance of these arcs. 



