Properties of the Alloys of Iron and Aluminium. 139 



At about 660° C. the curve develops a sharp point indicating 

 an abrupt increase in the permeability followed by an equally 

 abrupt decrease. This point will be discussed more fully in 

 Part II. 



It will be noticed that there are no sharp points in the 

 curves at the temperatures of maximum and minimum permea- 

 bility as might be expected to be the case if we were dealing 

 with a substance of uniform consistency. This general round- 

 ness suggests that the alloy is heterogeneous in structure, the 

 roundness being due to the different constituents reaching 

 their maximum and minimum values at slightly different 

 temperatures. 



A complete curve, showing all the changes of induction 

 with increasing temperature, is shown on fig. 11. 



The relatively great steepness of the curve between the tem- 

 peratures of 385° and 420° (about) is well shown in this figure. 



This general form of curve was found to be common to all 

 the alloys experimented on. 



In all cases there is a very sudden change in the induction 

 after passing the temperature of maximum induction, followed 

 by a flatness extending in the case of the alloy containing 

 10 per cent, of aluminium over a considerable range of 

 temperature, after which the curves again become steep. 



It was thought that it would be interesting to ascertain 

 whether the magnitude of this sudden change was in any 

 way connected with the strength of the field. 



To do this it is easy to select the temperature at which the 

 induction is a maximum, as this is fairly well marked, and is 

 about 380° C, The temperature at which the sudden change 

 ceases does not, however, appear to be the same for all fields. 

 The temperature 450° was arbitrarily chosen as that from 

 which to measure the termination of the sudden change. 

 The difference of B between these two temperatures is for 

 shortness referred to as ft. 



The following values were obtained for ft : — 



H. 



B at 380°. 



B at 450°. 



0. 



1222 



3860 



2960 



900 



7-39 



3375 



2520 



855 



4-09 



2630 



2030 



600 



3-12 



2130 



1650 



480 



220 



1300 



990 



310 



104 



300 



187 



113 



