Comparing Magnetic Fields

Let H₁ be the strength of the first field, let a magnet swing in it, and let the number of transits per second observed as in (b) be n₁, then we have, M being the magnetic moment,

Now let the magnet swing in the second field, strength H₂, and let n₂ be the number of transits per second. Then

Hence

To realise the conditions of this experiment surround the magnet hanging as in (a) with a soft-iron cylinder of considerable radius in comparison with the length of the magnet. The cylinder should be pierced with holes, through which the magnet maybe viewed, and the number of transits per second counted in the manner already described (p. 370).

The magnetic field within the iron cylinder is thus compared with that which the earth produces when the cylinder is removed.