Abstract
Permanent magnet, if once fully magnetized, retains magnetism permanently, but its intensity does not remain constant, and normally decreases gradually with elapse of time.
This change is known as ageing of permanent magnet, which is caused either naturally or by external disturbance, the latter case being the main subject of concern in this paper.
The external disturbance mentioned above can be classified into the following four types according to characteristics;
a) reluctance change of magnetic circuit.
b) application of outer field.
c) mechanical shock.
d) temperature change.
In the first three cases of a), b) and c), the general functional relations have been found between the change of the intensity and the number of repetition of the disturbance or its total repetition period. It proves that the change of the intensity increases in proportion to logarithm of the number of repetition or the total repitition period. Any general relation, however, has not been found in the case of d).
