Hysteresis area enclosed by a hysteresis curve (such as those in fig. 41) is a measure of the work done in carrying a cubic centimetre of the specimen through the corresponding mag netic cycle. This work, converted into heat, is known as the "hysteresis loss," and, expressed in ergs per cubic centimetre, is given by W = f Hdl = I f HdB.
The higher the maximum induction, the greater is the hysteresis loss. According to an empirical formula due to Steinmetz TV= 770.•°.
This formula holds very generally as a rough approximation.
Although the rise of temperature, even if no heat were lost, due to taking a specimen through a single cycle, would be small (a hysteresis loss of 5o,000 ergs for iron would correspond only to a rise of temperature of about .0014° C) when continuous rap idly alternating fields are applied, the rise in temperature and the waste of energy may be considerable. A knowledge of the hys teresis loss is of great importance from a technical point of view. It is necessary that for transformer cores, in particular, the loss should be as small as possible ; a material with a hysteresis curve of the second type shown in fig. 41 would be suitable for such a purpose.
There are various ways in which the hysteresis characteristics of a material may be briefly indicated :—by the value of the loss in ergs per cubic centimetre for some definite maximum induc tion (B=io,000 is often used) ; by the loss in watts per kilogram at 5o cycles per second; and by the Steinmetz coefficient 7). The hysteresis loss is greatest for cobalt magnet steels, and is progress ively less for other magnet steels, cast iron, low carbon steel, soft iron, electrolytic iron and certain iron-nickel alloys. For the par ticularly pure iron, which may be produced by electrolysis fol lowed by melting in vacuo, a loss of 813 ergs per cu.cm. (for 1o,o00) has been found, corresponding to 7)=.00032. A good quality soft annealed dynamo steel gave 7)=.00054, while for
cast iron, and hardened high carbon steels n has values of the order .on. For an iron-silicon alloy, particularly free from other substances, containing 3.4% Si, Yensen found a hysteresis loss of only 28o ergs (7)=.00010 ; in the best commercial silicon steels the losses are about io times Ps great. Silicon steel is much used for transformer cores. Its value lies in the fact that not only is the magnetic hysteresis loss small, but also the loss due to the heating effect of eddy currents, owing to the high value of the specific resistance of the material. Satisfactory silicon steel may be manufactured comparatively cheaply, as it is not necessary to use material which has been freed from carbon. The silicon ap parently hinders the carbon, which is ordinarily present, from entering into solution in the iron, and exercising its usual effect in increasing the coercivity and hysteresis loss.
Permanent Magnets.—For strong permanent magnets it is necessary to use a material for which the remanent magnetism (corresponding to the point E in the hysteresis curve of fig. 4o) is large. A large true remanence (as determined, for example, by the ballistic ring method) is not alone sufficient; for, when the external field is reduced to zero, a specimen in the form of a bar, or a horse-shoe, is subject to the demagnetizing field due to the poles at the end. The strength of a bar magnet depends on the apparent remanence, that is the magnetization in the de magnetizing field due to the bar itself. This field is proportional to the intensity of magnetization, and the demagnetizing factor, which is greater the shorter the magnet. (The demagnetizing fac tors for different ratios of length to thickness are given in the section on magnetic measurements in connection with the mag netometric method.) If the intensity decreases rapidly in an in