Resistance of the Electric Arc. 417 



from our cells. The sizes of the Apostle carbons were those 

 used by Mrs. Ayrton in her experiments. 



That the diameter of the carbons used does not have any 

 great effect on the resistance of the arc for the same current 

 and voltage is shown by the curves D and E, PL IV. and fig. 1, 

 Pl.V. These are the curves for two solid Apostle carbons — one 

 set 1) having carbons 11 millim. and 9 millim. and the other 

 set E having carbons 18 millim. and 5 millim. in diameter re- 

 spectively, and it will be seen that they lie very close together. 



On the curves each make of carbons is represented by a 

 particular kind of line as explained in the figure, the Apostle 

 carbons, for instance, are represented by a dotted line. Curves 

 bearing the same letter refer to the same combination of 

 carbons in the different makes. For each make there are four 



combinations: (-cored, —cored; + cored, —solid; -f solid, 



— cored ; -f solid, —solid; represented respectively by the 

 letters A, B, C, and D. 



The general characteristics of these curves are that the 

 ordinates of those for -f solid —solid are always negative, and 

 those for + cored —cored always positive, while the other 

 curves all lie between these two extremes, those which have the 

 positive carbon solid always being more negative than those 

 which have the positive cored. The greatest uniformity is met 

 with in the case of the -f solid —solid combination, in which 

 all the curves lie close together. For this reason the curves for 

 the Thorn son- Houston carbons have been omitted, as they lie 

 completely on the other curves, and would tend to cause 

 confusion. In the case of cored carbons, however, the uni- 

 formity is not so marked, as the material of the core varies 

 largely with the make. In fact, in a single carbon the core 

 may vary considerably, and for this reason the curves for 

 cored carbons are more troublesome to lake. 



PI. IV. shows the relation between the resistance of the 

 arc and the current that the arc is taking, the voltage at the 

 carbons being kept constant. It will be noticed that in the 

 case of those combinations which have a solid positive, the 

 number expressing the negative resistance of the arc increases 

 as the current decreases. With the other two combinations — 

 those having a cored positive — the curves for Brush and 

 Carre carbons have the same general characteristics, while 

 those for the Apostle carbons cut these latter. This is pro- 

 bably due to peculiarities in the coring of the Apostle 

 carbons. 



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 at 10 amperes in all cases. These curves are more 



