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Magnetic Hysteresis

If a magnetic substance is magnetized in a strong magnetic field, it retains a considerable portion of magnetism after the magnetic force has been withdrawn. The phenomenon of lagging of magnetization of induction flux density behind the magnetizing force is known as magnetic hysteresis.

Let a core of specimen of iron be would with a number of turns of a wire and current be passed through the solenoid. A magnetic field of intensity H pro-portional to the current flowing through the solenoid is produced. Let magnetizing force H is increased from zero to a certain maximum value and then gradually reduced to zero. If the values of flux density B in the core corresponding to various values of magnetizing force H are determined and B-H curves are drawn for increasing and decreasing values of magnetizing force H then it will be observed that B-H curve obtained for decreasing values of H lies above that obtained for increasing values of H.

Hysteresis Curve

While decreasing the magnetizing force H, when H is brought to zero the induction density B is represented by OC and is called as residual magnetism. The power of retaining the residual magnetism is called the retentively of the material.

Now if the direction of flow of current is reversed, the magnetizing force H is revered. Let the current be increased in the negative direction until the induction density H becomes zero. At this instant i.e. when B-0, the demagnetizing force H-OD, which is required to neutralize the residual magnetism, and is known as coercive force. If the demagnetizing force H is further increased to the previous maximum value and again gradually decreased to zero reversed and further increased in original of positive direction to the maximum value, a closed loop ACDEFGA is obtained which is usually known as hysteresis loop or magnetic cycle.

Hysteresis loops for hard steel, wrought iron and cast steel and for alloyed sheet steel are shown in.

Loop is for hard steel. Due to its high retentively power and large coercive force this material is well suited for permanent magnets. Since the area of hystersis loop for hard steel is large, therefore, hared steel is not suitable for rapid reversals of magnetization. Certain alloys of steel, aluminum and nickel known as alnico alloys are extremely suitable for permanent magnets,

Loop is for wrought iron and cast steel which rises steeply. Hence these materials have high magnetic permeability and good retentively, therefore, these materials are suitable for cores of electro-magnets.

Loop is for iron, low carbon steel, silicon, alloys, permalloy or Mumetal sheets. Since the permeability of these materials is very high and hysteresis losses are very low, there fore these materials are most suitable for transformer cores and armatures, which are subjected to rapid reversals of magnetization. Silicon alloys and permalloy (78.5% Ni ; 21% iron with small quantities of copper, molybdenum, chromium, cobalt and manganese etc.) are better for use as compared to  iron and low carbon steel.

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