CHAP. II] MAGNETIC CIRCUIT WITH IRON 25 



stant for the lower part of the curves. Such would be the case if 

 the lower parts of the B H curves were straight lines, as shown 

 in Figs. 2 and 3, because then the ratio of B to H would be con- 

 stant. However, in ordinary engineering work the lower parts of 

 magnetization curves are usually assumed to be straight lines, and 

 the permeability constant. 



Three parts can be distinguished in a B H or magnetization 

 curve: the lower straight part, the middle part called the knee of 

 the curve, and the upper part, which is nearly a straight line. As 

 the magnetic intensity H increases, the corresponding flux density 

 B increases more and more slowly, and the iron is said to approach 

 saturation. With very high values of the magnetic intensity H, 

 say several thousand ampere-turns per centimeter, the iron is com- 

 pletely saturated and the rate of increase of flux density with H is 

 the same as in air or in any other non-magnetic material. That is 

 to say, the flux density B increases at a rate of 1.257 kilolines for 

 each kilo-ampere-turn increase in H . Such is the slope of the upper 

 curve in Fig. 3. 



In view of this phenomenon of saturation the total flux density 

 in iron can be considered as consisting of two parts, one due to the 

 presence of iron, the other independent of it, as if the paths of the 

 lines of force were in air. These two parts are shown separately 

 in Fig. 4. The part OA, due to the iron, approaches a limiting 

 value B t , where the iron is saturated. The part OC, not due to the 

 iron, increases indefinitely in accordance with the straight line 

 law, B=ftH, where //= 1.257. The curve OD of total flux density 

 resembles in shape that of OA, but approaches asymptotically 

 a straight line KL parallel to OC. 



\\ hile it is customary to speak of the saturation in iron as being 

 low, high, or medium, the author is not aware of any generally 

 recogni/r<l method of expressing the degree of saturation numeri- 

 cally. It seems reasonable to define per cent saturation in iron with 

 respect to the flux density B 9 , so that, for instance, the per 

 saturation at the point N is equal to the ratio of PN' to B.. This 

 method of defining saturation, while correct theoretically, pre- 

 supposes that the ordinate B, is known, which is not always the 



<> percentage saturation of a mm- him- is defined in Art. r^ 

 tfindardization Rides of the Ai Institute nf i:i.dri. :il 



Engineers (edition of 1910) as the percentage ratio of OQ to PN, 



