#Magnetic analogue.#--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.