416 Messrs. J. Frith and 0. Rodgers on the 



Results. 



The amount of alternating current between the limits we 

 have used does not appear to have any influence on the value 

 obtained for the resistance of the arc. The largest alternating 

 current used had a R.M.S. (root mean square) value equal 

 to about 10 per cent, of the continuous current. The smallest 

 had about one-tenth of this value. In obtaining the curves 

 shown, the R.M.S. of the alternating current was usually 

 about 0*5 ampere. 



The frequency also did not have any effect on the resistance 

 of the arc between the limits 250 and 7 complete alterna- 

 tions per second. Frequencies above 150 were obtained 

 from a Ferranti alternator ; those between 150 and 45 from 

 a Pyke and Harris ; and those between 24 and 7 from a 

 Gramme machine. 



Besides these three machines a Mordey transformer of 

 ratio 1:1 was sometimes used, with its primary in circuit 

 with the Pyke and Harris alternator. On account of its 

 having a closed iron circuit it was not so suitable, but it 

 gave the same results as the alternators. 



We have thus obtained considerable variety in the wave 

 form as well as in the frequency. Hence the wave form 

 does not affect our measurements of the resistance of the arc. 



The values thus obtained for the resistance of the arc 

 under various conditions are plotted in Pis. IY. & V. PL IV. 

 shows the relation between the resistance of the arc and the 

 current, the P.D. between the carbons being kept constant. 

 Pl.V. fig. 1 shows the relation between the resistance of the arc 

 and the P.D. between the carbons, the current being kept 

 constant. Pl.V. fig. 2 shows the relation between the resistance 

 of the arc and the arc length, the current being kept constant. 

 In all these cases the resistance is measured from the central 

 line marked 0, positive values above the line and negative 

 values below the line. 



Certain makes of carbon have been selected for experiment, 

 namely, Apostle, Brush, Thomson- Houston, and Carre carbons, 

 as representative of the various qualities of carbon now in 

 use commercially. 



The diameter of the positive carbon was in all cases 

 11 millim., the diameter of the negative was 9 millim. in the 

 case of the Apostle and Brush carbons, 8 millim. in the case 

 of the Carre carbons, and 11 millim. in the case of the 

 Thomson-Houston carbons. These sizes were selected as 

 being most suitable for the currents we were able to take 



