RESEARCHES IN MAGNETISM. 
543 
curves is not inconsistent with the idea that is initially zero. By the time the 
dJQ 
magnetising force reaches the value of 1 c.g.s. unit or so, a pretty sharp upward bend 
takes place, and the magnetisation proceeds rapidly and at a nearly uniform rate for 
a considerable length of the curve. Then this rate changes gradually, as the region 
of what is popularly known as saturation is approached, in which region we again find 
a nearly straight portion of the curve, where a continuous though much less rapid 
increase of magnetism is still taking place. With magnetising fields whose strength 
is 100 units or so (my experiments did not deal with stronger fields) the values both 
of 23 and of 2> show no sign of reaching a limit. 
During the withdrawal of the magnetising force, if that has been strong, the 
magnetism stays well up in the region of so-called saturation until the force is almost 
wholly removed. As soon as a small force of opposite sign is applied the residual 
magnetism begins to disappear, and opposite magnetism to take its place, with great 
rapidity. The rate of this change with respect to the change of is however very 
uniform during the greater part of its progress. Other characteristics, such as the 
hysteresis which occurs when a magnetising force is removed and reapplied, have 
been already described, and others still will be evident on inspection of the curves. 
One feature which deserves special notice is the large amount of magnetism which 
even a very weak field will induce in annealed iron, when the specimen is endless, or 
long enough to prevent its ends from materially modifying the field. Take for 
instance the experiment last cited. With a field of 90 c.g.s. units the value of the 
induction 23 is 16,500, but to produce an induction of 10,000 a field of less than 
5 units is sufficient. And this susceptibility, high as it is, is greatly exceeded when 
during the application of the magnetising force the iron is subjected to mechanical 
vibration (see § 50). 
The curves showing the relation of magnetisation to magnetising force have a very 
different form when we are dealing with hard-drawn iron, or with a sample which 
instead of being in the annealed state, has been strained beyond its limit of elasticity 
before being magnetised. Examples of these will be given later ; meanwhile the 
following experiments with cast-iron and steel may be placed here for convenience of 
comparison with soft iron. 
§ 24. Magnetisation of a Cast-iron Ring. —The ring was turned circular, and of 
circular section, the dimensions being— 
Centims. 
External diameter.. . 15'05 
Internal „ .: 12 ‘30 
Diameter of section . . . . , . . 1'378 
The method of experiment was the step-by-step ballistic method of § 8. The results 
are shown in Plate 58, fig. 7. A large loop was formed by applying and reversing and 
reapplying a magnetising force of nearly 1G units ; and then a small loop was formed 
4 a 2 
