The Standard Electrical Dictionary

1. Magnetic Axis

   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.