In its improved form this meter consists of a single horseshoe permanent magnet formed of tungsten-steel having a strong and constant field.
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.
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.
By the use of a permanent magnet instead of a shunt coil as the bob of one pendulum, the meter can be made up as an ampere-hour meter.
It was also found that comparatively small magnets were sufficient, and that there was no particular virtue in the closed circuit and electromagnet, but that a small permanent magnet having one pole in contact with FIG.
A small permanent magnet is always liable to become demagnetized, or have its polarity reversed by the action of lightning.
Even a permanent magnet is susceptible of induction, its polarity becoming thereby strengthened, weakened, or possibly reversed.