Page:The New International Encyclopædia 1st ed. v. 12.djvu/770

* MAGNETISM. 688 MAGNETISM. lation is causfd to i)ass through a coil sur- rounding the spooinien, it is loinui that the value of B drops suddenly to the lower value, that is, the value on the ascending jjart of the curve. The electric oscillation sets up a molecular disturb- ance. This fact is made use of in the magnetic detector used in wireless telegraphy. The sud- den decrease in induction is used to induce an electric impulse in an au.xiliary circuit contain- ing a telephone. The eli'ect of hardening is shown in curve D, Fig. 1, which is taken from a test on the same sample of iron which gave the curve A after it had been subjected to a harden- ing process of rolling and .stretching. It will be noticed that the maxinuun values of B are lessened, the permeability is less throughout, the residual magnetism is less, the coercive force greater, and the area of the closed curve ap- ])reciably larger. When iron is subjected to such mechanical treatments as those mentioned above, and to annealing, hardening by iiuenchiiig. tempering, etc., the various resulting grades of steel have widely dillerent magnetic qualities aside from those due to dill'erences in cliemical composition. Speaking generally, a mild or soft .steel is also magnetically soft ;" that is to say, ^u is high and the coercive force low. The harder the steel the greater its magnetic hardness. This has bee)i already illustrated in the two curves given above. If two samples of steel differ in the amounts of carbon contained in them, the one having the greater amount is both mechanically and magnetically the harder, the permeability is lower, the coercive force higher. For this reason permanent magnets are made of steel. Also a specimen hardened by tempering is found to be much harder than one of the same chemical composition which has been annealed. Other substances than carbon affect the mag- netic quality of iron, sometimes very greatly; chromium and tungsten increase the coercive force tremendouslv. For this reason timgsten is generally used ::: magnet steel. The coercive force in soft iron is about 2, while that in tungsten steel may exceed 50. Cast iron reaches a somewhat lower magnetization than wrought iron or steel, even for high values of H. When saturated B is about three-quarters of the best values in iron. For moderate values of H in permeability and coercive force it generally re- sembles mild steel. In certain alloys of iron there is a marked absence of magnetic quality. The presence of manganese in large quantities is particularly detrimental. Thus in manganese steel, which contains about 12 per cent, of manganese and 1 per cent, of carbon, the permeability is only about 1.4, and is fairly constant in weak and strong fields; also, there is practically no lesid- ual magnetism. Nickel steel is also most re- markable. A specimen containing 25 per cent, of nickel was foimd to be practically non-mag- netic iinder ordinary conditions of temperatur'?, its permeability being ju'actically constant at 1.4. Thus we have an alloy of two metals, each itself strongly magnetic, which has a practical absence of all magnetic quality. This alloy is also interesting in the further fact that when cooled to very low temperatures it becomes strongly magnetic and remains so after the temperature rises to ordinary values. The effect of increase of temperature generally is to increase the magnetic properties of iron when the magnetizing force is low. This increase continues up to a temperature of 775° C, and beyond this temperature the iron suddenly be- comes practically non-magnetic. This tempera- ture is known as the critical temperature of magnetization, and the evidence is plentiful from other fSicts that there is a decided molecular change in the structure of the iron at this point. For instance, this is the region known as the point of recaleseence in the cooling of iron from white heat. The suddenness of this loss of mag- netic quality with temperature is less as the magnetizing force is greater, and for large values of H it may even liappen that the permea bility decreases with increase of temperature. 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 case in electrical apparatus, this loss maj' 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, a JOOOO / ^^ 6000 / y^ 6000 ,> / y^ ^^,_--^ 4000 / / fCy--^ 2000 /// I 2 3 Watts per pound Fig. 3, CURVES showing losses of energy is iron. however, and in alternating current apparatus in which the induction rapidly changes direction, the loss may be quite great and it directly af- fects the efficiency of the machine. (See Dy- n.mo-Electbic Machinery.) Hence, in these latter cases, the iron is worked at lower indvu tion and must 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 W = ?/ B'-". W is the loss in ergs pi r cycle, B is the maximum induction, and v is .i 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 ns .015 for steel and .08 for tungsten and manganesi- steel. Thus at 00 evcles per second if the induc- tion be GOOO, for n — -0033 the loss in 100 cu. cms. would be 2.1 X 10' 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 aa to cut down the number of closed circuits in the body of the iron. In measuring iron losses due