Resistance of the Electric Arc. 419 



Particulars of Cores. 

 The diameters of the various cores are as follows : — 





Diam. 



Diam. 



Carbon. 



of carbon. 



of core. 





mm. 



mm. 



Apostle 



. ... 11 



2-7 





9 



2*33 



Brush 



. ... 11 



2-73 





9 



2-18 



Carre 



. ... 11 



24 





8 



1-9 





Inverted Arcs. 





In the experiments hitherto described the positive carbon 

 has always been uppermost. Experiments were now made to 

 ascertain if the resistance of the arc is dependent upon 

 whether the positive carbon is the upper or the lower. 



Using solid carbons, the resistance is not appreciably 

 altered by inverting the arc. With cored carbons, however, 

 the resistance of the arc is altered, and moreover the physical 

 character of the arc is changed. As we have pointed out 

 above, the distinguishing feature of the arc when using cored 

 carbons is the appearance of a dark space situated near the 

 centre of the purple glow, which appears to considerably 

 increase the resistance of the arc. On inverting such an arc, 

 however, we find that this dark space completely disappears, 

 and, as we should have expected, the resistance is consider- 

 ably diminished. At low currents or P.Ds., the difference 

 between the resistances of ordinary and inverted arcs dimi- 

 nishes till the two values become nearly equal. These are 

 the conditions under which the dark space ordinarily dis- 

 appears. Hence when the dark space has disappeared, 

 inverting the arc makes but little difference in its resistance, 

 which is also the case with arcs using solid carbons, in which 

 there is no dark space. 



By placing the carbons horizontal, a value for the resist- 

 ance of the arc can be obtained which is intermediate between 

 the two values obtained by placing the positive carbon 

 respectively above and beneath the negative carbon, the 

 current and P.D. being the same in the three cases. 



From consideration of the part played by the dark space, it 

 would appear that the resistance of the arc is greatly affected 

 by the state of the contact between the purple glow and the 



