Electrical Engineering ⇒ Topic : Residual flux density

 Retentivity of the material.

If now H is gradually reduced (by reducing solenoid current), it is found that the flux density B does not decrease along the same line by which it had increased but follows the path ab. At point b, the magnetising force H is zero but flux density in the material has a finite value + B_r (= ob) called residual flux density. It means that after the removal of H, the iron piece still retains some magnetism (i.e.+B_r). In other words, B lags behind H. The greater the lag, the greater is the residual magnetism (i.e. ordinate ob) retained by the iron piece. The power of retaining residual magnetism is called retentivity of the material.

The hysteresis effect (i.e. lagging of B behind H) in a magnetic material is due to the opposition offered by the magnetic domains (or molecular magnets) to the turning effect of magnetising force. Once an-anged in an orderly position by the magnetising force,the magnetic domains do not return exactly to the original positions. In other words, the material retains some magnetism even after the removal of magnetising force. This results in the lagging of B behind H