.#--Let us consider the effect that a magnetising force
produces on a bar of soft iron. It is known that each molecule in such a
bar is an individual magnet. The bar as a whole, nevertheless, exhibits
no external magnetisation. This is held to be due to the fact that the
molecular magnets are turned either in haphazard directions or in closed
chains, and there is therefore no resultant polarity. But when the bar
is subjected to a magnetising force by means, say, of a solenoid
carrying electrical current, the individual molecules are elastically
deflected, so that all the molecular magnets tend to place themselves
along the lines of magnetising force. All the north poles thus point
more or less one way, and the south poles the other. The stronger the
magnetising force, the nearer do the molecules approach to a perfect
alignment, and the greater is the induced magnetisation of the bar.

The intensity of this induced magnetisation may be measured by noting
the deflection it produces on a freely suspended magnet in a
magnetometer.

The force which produces that molecular deflection, to which the
magnetisation of the bar is immediately due, is the magnetising current
flowing round the solenoid. The magnetisation, or the molecular effect,
is measured by the deflection of the magnetometer. We may express the
relation between cause and effect by a curve in which the abscissa
represents the magnetising current, and the ordinate the magnetisation
produced (fig. 82).

[Illustration: FIG. 82.--CURVE OF MAGNETISATION]

In such a curve we may roughly distinguish three parts. In the first,
where the force is feeble, the molecular deflection is slight. In the
next, the curve is rapidly ascending, i.e. a small variation of
impressed force produces a relatively large molecular effect. And
lastly, a limit is reached, as seen in the third part, where increasing
force produces very little further effect. In this cause-and-effect
curve, the first part is slightly convex to the abscissa, the second
straight and ascending, and the third concave.

#Increase of response with increasing stimulus.#--We shall find in dealing
with the relation between the stimulus and the molecular effect--i.e.
the response--something very similar.

On gradually increasing the intensity of stimulus, which may be done, as
already stated, by increasing the amplitude of vibration, it will be
found that, beginning with feeble stimulation, this increase is at first