24 THE MAGNETIC CIRCUIT [ART. 12 



while three different scales are used for the carbon-steel curve. 

 The values of H at very low flux densities are unreliable because 

 in reality each curve has a point of inflexion near the origin, not 

 shown in Figs. 2 and 3 (see Fig. 7). 



The curves given in Figs. 2 and 3 represent the averages of many 

 curves obtained from various sources. The iron used in an indi- 

 vidual case may differ considerably in its magnetic quality from the 

 average curve. The value of B obtainable with a given H depends 

 to a large degree upon the chemical constitution of the specimen, 

 impurities, heat treatment, etc. As a rule, the soft and pure grades 

 of steel are magnetically better, that is to say, they give a higher 

 flux density for the same magnetizing force, or, what is the same, 

 they possess a higher permeability. Annealing improves the 

 magnetic quality of iron, while punching, hammering, etc., lowers 

 it. Therefore, the laminations used in the construction of elec- 

 trical machinery are usually annealed after being punched into 

 their final shape. This annealing also reduces hysteresis loss. 



12. Permeability and Saturation. Permeability is defined in 

 Chapter I as the permeance of a unit cube, or, according to eq. 

 (16), as the ratio of B to H. The two definitions are, of course, 

 identical. Therefore, the values of permeability for various values 

 of B are easily obtained from the magnetization curves. For 

 instance, for cast steel, at B= 15 kilolines per sq. cm. the magnetic 

 intensity H is 26 ampere-turns per cm., so that /*= 15000/26 = 577 

 perms per cm. cube. This is the value of the absolute permeabil- 

 ity in the ampere-ohm-maxwell system. In most books the relative 

 permeability of iron is employed, referring to that of the air as 

 unity. Since in the above-mentioned system /*=1.25 for air, 

 the relative permeability of cast steel at the selected flux density 

 is 577/1.25 = 461. 



In practice, the calculations of magnetic circuits with iron are 

 arranged so as to avoid the use of permeability /JL altogether, using 

 the B H curves directly. In some special investigations, how- 

 ever, it is convenient to use the values of permeability, and also 

 an empirical equation between // and B. For instance, see the 

 Standard Handbook for Electrical Engineers-, the topic is indexed 

 " permeability curves/' and " permeability equation." These 

 /z B curves show that there must be a point of inflection in 

 the BH curves at low densities, because the values of /* reach 

 their maximum at a certain definite density instead of being con- 



