274 Dr. Fleming, and Messrs. Ashton and Tomlinson on 



corresponding to a certain small value of log 10 B ; and 

 accordingly it follows that the hysteretic constant w, which is 

 represented by the tangent of the inclination of the line 

 (log W, log B) cannot have the value 1*6 throughout all 

 values of B max ., but must increase as B max , decreases. That 

 this is also the case for this cobalt sample is well shown by the 

 lowest observation-points on the logarithm-curve (fig. 3). We 

 conclude from the fact that the hysteretic exponent attains a 

 higher value than 1*6 for very low inductions, that there is 

 in cobalt also a non-hysteretic range of cyclical magnetization. 

 It is usual to express W in terms of B; but inasmuch as W 

 is really an expression for the behaviour of the metal per se 

 corresponding to a given state of magnetization, it would be 

 better to express W in terms of the magnetization I. Of the 

 three magnetic vectors H, B, and I, I has reference to the 

 properties of the material itself, B to those of the material 

 and the space it occupies as well, and H may be looked upon 

 as denoting the space qualities only. 



Hence magnetic material qualities like W should be expressed 

 in terms of I rather than B. 



We have accordingly given the logarithmic curve con- 

 necting log 10 W and log 10 I, I being the maximum magneti- 

 zation during the cycle. 



The fact that the hysteresis loss W during a cycle can be 

 approximately expressed in terms of the maximum magneti- 

 zation I during the cycle, seems to show that the work done 

 in making a complete magnetic cycle, or carrying the mag- 

 netic molecules once completely round, is nearly a simple 

 exponential function of the percentage of them which are 

 collineated at the beginning and end of the cycle in the 

 direction of maximum magnetization. The magnetization 

 being the magnetic moment per unit volume, it follows that the 

 work done in making a complete magnetic reversal of all the 

 molecular magnets is a nearly simple exponential function of 

 the total resolved molecular magnetic moments in the direction 

 of magnetization at the beginning and end of the operation. 



It appears, therefore, that the work done in carrying the 

 magnetic molecules in unit volume once round a complete 

 cycle is nearly proportional to the l"6th power of the aggre- 

 gate magnetic moment of all the molecules estimated in the 

 direction of the magnetic force. 



The more the molecular magnets are collineated, that is, the 

 greater the aggregate magnetic moment in a given direction, 

 the greater the work required to effect a complete cyclic ope- 

 ration, the two magnitudes being related by a simple expo- 

 nential relation. It would be interesting to determine if the 



