We have seen that the area of the loop of hys teresis represents a loss of energy. This fact is of no special importance if the magnetization is constant and in one direction; but if the mag netization varies, or if it reverses many times per second, as is frequently the ease in electrical apparatus, this loss may be quite appreciable. Thus in the magnetic circuits of dynamo-electric machines the magnetization in the fields is al ways in one direction, and hence wrought iron of high permeability is used. • In the armatures, however, and in alternating current apparatus in which the induction rapidly changes direction, time loss may be quite great and it direetly af fects the efficiency of the machine. (See DY NAMO-ELECTRIC Hence, in these latter cases, the iron is worked at lower induc tion and tnust be soft so as to give a narrow hysteresis curve. Steinmetz has expressed a convenient law for this loss. it is stated in the formula = 7/ 13'•°. W is the loss in ergs per cycle. B is the maximum induction, and i is a constant of the material ; 7/ varies from .00124 to .0055 and for good annealed iron has an average value of .0033. It may be as high as .015 for steel and .08 for tungsten and manganese steel. Thus at 60 cycles per second if the induc
tion be 6000, for e = .0033 the loss in 100 cu. ems. would be 2.] X 10T ergs per second or 2.1 watts. Closely allied with the hysteresis loss for alternating magnetization is the so-called Foucault or eddy current loss. When the in duction alternates it induces currents in numer ous little closed paths in the body of the iron due to its electrical conductivity. These cur rents. therefore, cause a loss of energy. To di minish these losses the iron is laminated so as to cut down the number of closed circuits in the body of the iron. In measuring iron losses due to alternating magnetization it is very difficult to separate these losses. In practice they are measured together and the curves shown in Fig. 3 give the loss per pound of good soft sheet steel .011 inch thick at different inductions for 25, GO, and 133 cycles per second.
BIBLIOGRAPHY. The books of reference menBibliography. The books of reference men- tioned in the article ELECTRICITY Will also suf fice for the subject of Magnetism, while in addi tion Ewing, Magnetic Induction (London, 1900), and DuBois, The magnetic Circuit in Theory and Practice, translated by Atkinson (London, ISM), are to be recommended.