The magnetic properties of iron vary widely for different specimens. These variations depend in the first place on the past history of the material; that is to say, whether it has been annealed, hardened, stretched, twisted, or sub jected to any treatment which would be likely to affect the molecular structure. In the second place, they depend on the chemical composition; that is to say, the presence or absence of such substances as manganese, carbon, etc. In general it may be stated that the softer the iron the greater the values of B and g which may be obtained. The curve A, Fig. 1, is taken from a test on an average specimen of good soft iron. The maximum value of B is about 15,000. the maximum A about 245, corresponding to 11=4.5. The whole curve in this case is unusually nar row, and therefore incloses a small area, showing that in soft iron B follows H with greater readiness than in harder specimens. and that the loss due to hysteresis is smaller. When sub jected to a magnetizing force the behavior of soft iron is particularly sensitive to any kind of mechanical disturbance. Thus if the specimen be lightly tapped while any given value of 11 is applied, the corresponding value of B may be largely increased. It retains considerable resid ual magnetism if undisturbed, but this residual practically disappears if the iron is tapped or heated, or the molecular structure is disturbed in any way. If soft iron is mechanically hard ened in any way, that is to say. if it is ham mered. rolled, stretched, or twisted. its per meability and value of residual magnetism are much lessened, and the coercive force is in creased.
If, while the iron is on the descending part of the hysteresis curve and so has the higher value of B for any given value of H, an electric oscil lation is caused to pass through a coil sur rounding the specimen. it is found that the value of B drops suddenly to the lower value, that L.-, the value on the ascending part 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 auxiliary circuit contain ing a telephone. The effect of hardening is shown in curve D, Fig. I, 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 maximum 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 preciably larger.
When iron is subjected to such mechanical treatments as those mentioned above, and to annealing, hardening by quenching. tempering. etc., the various resulting grades of steel have widely different magnetic qualities aside from those due to differences in chemical composition.
Speaking generally, a mild or soft steel is also magnetically soft; that is to say, /.4 is high and the coercive force low. The harder the steel the greater its magnetic hardness. This has been 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 m1111(.11 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 tremendously. For this reason tungsten is generally used in 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 II. When saturated 1; is about 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 partieularly detrimental. Thus in manganese steel. which contains about 12 per cent. of manganese and 1 per cent. of carbon, time permeability is only about 1.4, and is fairly constant in weak and strong fields: also, there is practically no iesid ual magnetism. Nickel steel is also most re markable. A specimen containing 23 per cent. of nickel was found to be practically non-mag netic under ordinary conditions of temperature, its permeability being practically 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 lu•e is known as the critical temperature of magnetization, and the evidence is plentiful from other facts 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 recalescence 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 happen that the permea bility decreases with increase of temperature.