Resistance of the Electric Arc. 415 



Thick wire coil. 



Size of wire No. 12 S.W.G. 

 Number of turns . . . . = 128. 

 Kesistance =0*143 ohm. 



Section No. 2 : 



Fine wire coil. 



Diameter of wire (uncovered) =3 mils. 



„ „ (covered) =5 „ 



Number of turns .... =29,500. 



Resistance =32,120 ohms. 



Thick wire coil. 



Size of wire No. 12 S.W.G. 

 Number of turns .... = 96. 

 Resistance =0*112 ohm. 



The thick wire was double-cotton-covered. The thin wire 

 was silk-covered. 



The voltmeter was an Ayrton-Mather reflecting electro- 

 static voltmeter giving a deflexion of 700 scale-divisions at a 

 distance of 2500 scale-divisions for 100 volts. This instrument 

 was admirably adapted to our purpose on account of its very 

 quick swing and excellent damping. 



The resistance R consisted of a box wound with thick 

 german-silver wire plugging to 0*1 ohm, in series with which 

 was a german-silver wire (No. 10 S.W.G.) doubled back on 

 itself and carrying a slider. Points were marked out at dis- 

 tances corresponding to 0*01 ohm. These spaces were about 

 10 7 centims. long. 



The Alternator. Several different alternators were used. 

 The one used while obtaining the curves given was a Pyke 

 and Harris machine, which was especially suitable, since it 

 had no sliding contacts, both armature and field being fixed. 

 The other machines were — for high frequencies a Ferranti, 

 and for low frequencies a Gramme alternator. The fields 

 in all cases were excited by storage-cells. 



The condenser had usually a capacity of about 60 micro- 

 farads. 



Variation of Conditions. 



The conditions of the experiments were varied as much as 

 possible. We have studied the effect on the resistance of the 

 arc of variations in the amount, frequency, and wave-form of 

 the alternating current ; the effect of different kinds of 

 carbons and different P.Ds. and currents ; the effect of using 

 different combinations of cored and solid carbons, of carbons 

 cored with substances other than carbon ; and the effect of 

 the relative size of the carbons. 



