1892.] Mr Searle, On a compound magnetometer. 



331 



values which H has previously assumed. Thus if the magnetising 

 force is made to undergo a series of changes in a cyclical manner, 

 the curve representing the relation of / to H will be of the form 

 of a loop, which however degenerates into a straight line when 

 the maximum value of H does not exceed 04c.G.s. units*. This 

 dependence of the intensity of magnetisation, due to a given 

 magnetic force, upon the previous magnetic history of the iron 

 has been called by Ewing hysteresis. The curve ABCDEFA (fig. 1) 



Hysteresis Curve for Annealed Steel Wire. 



taken from Prof. Ewing's book on " Magnetic Induction in Iron 

 and Other Metals " will serve to give a general idea of the relation 

 between / and H for a piece of annealed pianoforte steel wire 

 when the magnetic force is made to pass repeatedly through a 

 complete cycle of changes. The maximum values of H and / in 

 this curve are about 100 and 1100 C.G.S. units respectively. The 

 curve OB gives the relation between / and H for a piece of steel 

 which has never previously been magnetised or which has been 

 completely demagnetised by continued reversals of a magnetic 

 %force whose amplitude has been slowly diminished to zero. 



These hysteresis curves are of great interest from the practical 

 as well as from the philosophical point of view, since, as has been 

 shown by Warburg and by Ewing-f-, the area of the curve, when 

 estimated on the proper scale, represents the energy expended 

 per cubic centimetre of the iron in carrying H through its cycle 

 of changes. 



In determining the form of the hysteresis curve, a specimen 

 of the material in the form of a wire is placed inside a long 

 uniformly wound solenoid through which a current can be sent. 

 The current gives rise, inside the solenoid, to a uniform longi- 

 tudinal magnetic force whose value can be calculated from the 



equation 



H = 4fTTlU, 



* Lord Eayleigh, Phil. Mar,. March, 1887. 



t J. A. Ewing, "Magnetic Induction in Iron and Other Metals," § 79. 



VOL. VII. PT. VI. 2G 



