JAN. 19, 1921 proceedings: philosophical society 39 



p = a + /3// (1) 



but for high inductions this has to be modified to 



H 



B - H 



= «, + m (2) 



where p^ is the metalHc reluctivity and B — H is the metalhc induction or 

 flux carried onl}' 1)y the molecules of the metal. For pure and well-annealed 

 materials the reluctivity line is a straight line while in the more or less im- 

 pure commercial materials the reluctivity line while approximately a straight 

 one has generally a point where its slope changes. Since the change in the 

 slope of the line is in general greater with the increase in impurity of the 

 material the cause is evidently a lack of homogeneity; /. e., the presence in 

 the substance as aggregates or inglomerates of materials of different mag- 

 netic characteristics. Although the arrangement of the constituents in a 

 carbon steel is generally a random one, it may be considered as combinations 

 of any of the three simple arrangements; viz., (a) the parallel, (h) the series, 

 and (c) the spheroidal. 



In the experiments described in the present paper, two specimens of an 

 eutectoid carbon steel (0.S5 per cent carbon) were selected and turned down 

 to a uniform diameter of approximately 7 mm. Each of these specimens 

 was quenched from iSO()° C, one in oil, the other in water, and then care- 

 fully groimd down to (> mm. Each specimen was then cut in halves desig- 

 nated A and B, which were then drawn alternately to increasingly higher 

 temperatures up to 700° C. Normal induction curves, up to values of the 

 magnetizing force as high as 2500 gauss, were determined after each op- 

 eration. 



Curves for a few of these heat treatments were shown. Other magnetic 

 properties shown graphically were (a) the values of the induction for con- 

 stant values of the magnetizing force as plotted against the drawing tem- 

 peratures; (6) the reciprocals of the susceptibility (47r times the reluctivity) 

 as plotted against the magnetizing forces; (c) the value of the saturation 

 intensity of the magnetization as plotted against the drawing temperatures ; 

 and {d) the values of the "magnetic hardness" as plotted against the draw- 

 ing temperatures. Referring to equation (2) the values of the saturation 



intensity of magnetization are equal to — -- ; i. e., the reciprocal of the slope 



47rp 



of the reciprocal of susceptibility line. The values of the "magnetic hard- 

 ness" are the values of a; i. c, the intercept on the axis of ordinates. 



These two characteristics are those with which this paper is most concerned, 

 since they show more than any others the transformations which take place 

 in the steel as a result of the heat treatment. It is found that in a region 

 included by the quenched condition and a drawing temperature of 230° C, 

 in which the specimen is known to be martensitic in structure there is a 

 distinct bend in the reluctivity lines and consequently there are two values 

 of the saturation intensity of magnetization and the "magnetic hardness" 

 as calculated from the upper and lower portions of the reluctivity line. In 

 the range of drawing temperatures bounded by 230° C. and 470° C, in which 

 the steel is troostitic, the reluctivity line has but one slope, hence the satura- 

 tion intensity of magnetization and the "magnetic hardness" each have but 

 one value for a given drawing temperature and the material is magnetically 

 homogeneous. Beyond 470° C. the reluctivity line again has a bend and 



