on the Electric Arc. 13£ 



are below the curves for Fe-Cu and C-Cu when the arcs 

 are in air. But they are above when the arcs are in 

 hydrogen. With carbon for the negative, copper's place is 

 seen to be above iron in both air and hydrogen. 



It was desired to obtain a comparison of the slopes of the 

 curves for C-C in air and inelal-0 and G-C in hydrogen, in 

 order to show the change in voltage for a given change in 

 current in the several cases. All experiments agreed in the 

 result shown in fig. 10, namely, that the C-C curve in air 

 cuts the other curves in the neighbourhood of Q'b amperes 

 and 47 volts, and is a much flatter curve. In fact, all the 

 air-curves derived are flatter than those in hydrogen with the 

 exception of that for C-Al. This arc is difficult to measure, 

 but very careful observations led to the conclusion that the 

 curve was, if anything, steeper than as shown in fig. 5. 



Curves were also taken of the copper-copper arc in air 

 under three conditions : — 



(1) -1- above, and cooled by water-circulation. 



(2) — above, and cooled by water-circulation. 



(3) Arc between two rods of 0'375 in. diam., with 



4- above, and no cooling. 



The latter curve lies between the others, but is somewhat 

 steeper, as shown in fig. 11. 



Voltage Equations of different Arcs. 



In determining the equations of different arcs, the form 

 of procedure given by Mrs. Ayrton has been followed. The 

 characteristics have first been sketched in according to the 

 readings obtained. From these curves were drawn up 

 tables of volts, amperes, and watts. The watts were then 

 plotted against amperes. This should give straight lines if 

 the volt-ampere curves are satisfied by equations of the form 



V • h 



1 = a "*" A' 



where a and b are constants, 1 = length of arc, and A = current 

 in amperes. In all cases, the approximation to a straight 

 line was so close, that no doubt could be entertained that the 

 deviation was due to error of observation. 



From these watt-ampere curves new sets of tables were 

 drawn up, giving volts, and from these derived volts and 

 amperes the curves shown in the different figures have been 

 drawn. The derived curves have in all cases been found to 

 satisfy the original points, and may be regarded as more 



