620 Prof. Richardson and Mr. Lownds on the Magnetic 

 where K 1; K 2 are the two values of K for the two curves con- 



sidered, or 



B 



= (K x — K 2 ) = con stant. 



Hence, on the ahove assumption, the curve connecting 

 (Hj — Hg) and B should be a straight line. This curve is 

 shown as III on fig. 4. 



It will be seen that this curve differs considerably from a 

 straight line. It is interesting, however, to compare these 

 curves with some obtained by Mr. C. G. Lamb (Phil. Mag. 

 Sept. 18-99). Mr. Lamb experimented on a piece of iron first 

 in the form of a bar, and secondly in the form of a ring. 

 The B and H curves for the ring and bar are shown in 

 Mr. Lamb's paper on fig. V. as R and P respectively. 



If now H 1? H 2 are the field-strengths corresponding to a 



r en value of B for these two curves, then 



H, -H 



B 



- should 



theoretically be a constant, and the curve connecting (Hi — H 2 ) 

 and B should be a straight line. 



If, however, a curve connecting (Hj — H 2 ) and B be 

 plotted from the two sets of observations in question, it will 

 be found to differ from a straight line, and to be very similar 

 in general form to curve III above. Hence it is concluded 

 that the assumption made to explain the change in the per- 

 meability of the alloy due to repeated heating and cooling is 

 probably much nearer the truth than the form of curve III 

 would at first sight seem to indicate. 



§ YIL Experiments near the Temperature of Minimum 

 Permeability. 



A third set of experiments w r ere undertaken to trace the 

 changes in the induction (with a constant field) with change 

 of temperature in the neighbourhood of the temperature of 

 minimum permeability. 



As mentioned in Part I., an abrupt change in the per- 

 meability is observed at a temperature somewhat less than 

 this. 



This abrupt change was more marked during cooling than 

 during heating. It attained its maximum value at a tempe- 

 rature of 652° C. with rising temperatures, and at a tempera- 

 ture of 645° C. with falling temperatures. 



This small difference of 7° 0. might be due to the tempe- 

 rature of the ring lagging behind the temperature of the 



