The line connecting the poles of a magnet. It does not generally
coincide exactly with any symmetrical axis of figure. In such cases an
error is introduced into the indications of the needle which must be
determined and allowed for in compasses. To determine it with a magnetic
needle the suspension cup is made removable, so that the needle can be
reversed. Readings are taken with one side of the needle and then with
the other side of the needle up, and the average corresponds with the
position of the magnetic axis in both positions of the needle.
