Question

what is the difference between the real earth and the magnetic length of a magnet​

Answer 1

The actual length of a magnet is called the geometric length of the magnet. The distance between the poles of a magnet is called the magnetic length of the magnet. The geometric length of the magnet is nearly 8/7 times the magnetic length of the magnet.

Answer 2

Answer:

Magnetic length is a mostly out of date concept. It’s relevant for magnetic materials with low coercivity because permanent magnets tend to demagnetize themselves and a low coercivity magnet will demagnetize itself more.

Also because of low coercivity, magnets made with old alloys such as AlNiCo need to be much longer in their easy axis (the magnetized direction) than the hard axes, which results in apparent shortening of the distance between the poles.

With modern rare earth magnets that have much higher coercivity (typically 10x or more) that shortening is much less. Also because of their higher coercivity they are most effective when their easy axis is the short dimension, reducing the difference between the magnetic length and geometric length even more.

So the concept of “magnetic length” is no longer used with modern magnetic materials.