Hysteresis

Hysteresis is a physics term that means, literally, to be late. It describes systems that do not directly follow the forces applied to them, but react slowly, or don't return completely to their original state. For instance if you push on a piece of putty[?] it will assume a new shape, and when you remove your hand it will not return to its original shape, or at least not entirely.

The term is actually used almost entirely to describe an effect seen in magnetism, specifically in ferromagnetic materials. When an external magnetic field is applied to a ferromagnet, the ferromagnet "takes up" some of the external field. Even when the external field is removed, the magnet will retain some field, it has become magnetized.

A hysteresis loop occurs when an alternating magnetic field is applied to a ferromagnetic material. As the magnetic field increases, the magnetization also increases, but it has a lower value than the equilibrium magnetization. The magnetic field then reverses direction and starts to decrease - again the magnetization lags, this time staying above the equilibrium value. If the field is plotted against magnetization during this cycle, it will appear as a loop.

We sorely need a diagram here. Even a freehand sketch would be better than nothing.

This is a very important effect in many situations, notably magnetic tape and other related storage media like hard disks. In these materials it would seem obvious to have one polarity represent a bit, say north for 1 and south for 0. However if you want to change the storage from one to the other, the hysteresis effect requires you to know what was already there because the needed field will be different in every case.In order to avoid this problem, recording systems first overdrive the entire system into a known state, a process known as bias. Different materials require different biasing, which is why there is a selector for this on the front of most cassette recorderss.

The term "hysteresis" is sometimes used in other fields, for example economics. In such cases it describes some sort of memory or lagging effect.

External links:

Hysteresis (http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst)

What's Hysteresis? (http://www.lassp.cornell.edu/sethna/hysteresis/WhatIsHysteresis)