456 Mr. "W. E. Sumpner on the Variation 



C 



and if Ave multiply this by -- — 



we shall obtain the value 



of 7^ — -^, or the tangent of the angle which the chord PxP 2 



O x — b 2 

 makes with the line OC. 



By observing the ratio — ^ — -, in addition to the ordinary 



quantities measured in Maxwell's method, it is possible to 

 determine in this way the rate of slope of the curve of mag- 

 netization. If (Cx — C 2 ) is very small compared with 

 i(Ci + C s ), the value obtained by Prof. Ayrton's method 

 is Lj; but the result is practically the same if the curve of 

 magnetization does not bend very much between the values 

 of OC corresponding with C x and C 8 . 



Coefficients for small Currents. 



4. It follows from the shape of the initial part of the curve 

 of magnetization that the value of L 2 must at first increase with 

 the current. The following experiments show that this is the 

 case. A bar of best Swedish iron, 14 in. long by \ in. 

 in diameter, was bent into the shape of a horseshoe. Each 

 end was surrounded by a bobbin, 2i in. in diameter and 

 4^ in. long, wound with 400 turns of wire. The self-induction 

 of the two bobbins in series was obtained by Maxwell's method. 

 The battery used was an ordinary Leclanche cell. The swings 

 were in most cases taken on breaking the circuit, since, if the 

 galvanometer-key be pressed just before the battery-key is 

 released, the error caused by a slight want of balance in the 

 bridge is reduced to a minimum. For a similar reason, the 

 deflection corresponding with a known derangement was 

 always estimated from the difference between two deflections. 

 The current flowing through the coil was calculated from the 

 electromotive force of the battery and the resistances in the 

 network. The following numbers represent the mean of 

 several concordant values : — 



A. 



L 2 . 



L,-2A. 



•047 



•0514 



•0420 



•056 



•0539 



0428 



•0(50 



•0649 



•0429 



•Olio 



•0554 



0424 



•068 



•0564 



•0428 



•079 



•0577 



•0423 



•091 



■0609 



■0427 



107 



•0634 



■0420 



In this table L is the coefficient of self-induction of the 



