A small permanent magnet is always liable to become demagnetized, or have its polarity reversed by the action of lightning.
This liability is overcome by making such movable parts as require to be magnetic of soft iron, and magnetizing them by the inducing action of a strong permanent magnet.
A much better form of electromagnetic ammeter can be constructed on a principle now extensively employed, which consists in pivoting in the strong field of a permanent magnet a small coil through which a part of the current to be measured is sent.
It is essential that the permanent magnet should be subjected to a process of ageing so that its field may not be liable to change subsequently with time.
The wire will in fact become temporarily magnetized by induction, that end of it which is nearest to the pole of the magnet acquiring opposite polarity, and behaving as if it were the pole of a permanent magnet.
