482 Mr. L. Southerns on the 



q 2 

 value—, but the law will alter for such wide separation. 



Actually this value will be reached when the fibres are 



k 2 

 parallel. When // is reduced to — the sensitiveness becomes 



infinite. P 



Such a curve as that given in fig. 4 may be plotted from 



the results of experiment. Distances Oh will not be known 



accurately, but if the ordinates be plotted against differences 



of O/i, which may be taken to be proportional to rotations of 



the adjusting screw, the position of 0' may be found by 



, q 2 

 graphical means, thus giving the value of — and al.^o of the 



a 2 k 2 k 2 P 



constant — — from which — ? and thus the position of 0, can 

 p- p, p 



be determined. 



Now suppose the balance to be made less sensitive by 

 lowering the centre of gravity of the beam. This corre- 

 sponds to an increase in r' the distance which w must be 

 raised to give infinite sensitiveness. This means an alteration 

 of the centre 0' in fig. 4, both its co-ordinates being reduced 

 in the same ratio, as reference to (6) will show. Thus the 

 locus of 0' is a straight line through 0. This fact affords 

 another means of obtaining the position of 0, and the 

 absolute horizontal distance between P and Q. 



Fig. 5 gives a series of curves obtained by experiment on 

 a Curie balance hastily fitted with roughly adjustable fibres 

 carrying a small weight. Deflexions of the beam were read 

 by means of the microscope which is permanently fitted to the 

 balance. Ordinates of the curves represent deflexions for a 

 small weight added to one scale-pan, and abscissae rotations 

 of the adjusting screw of the fibres. The curves are not 

 perfect hyperbolas, but deflexions in some cases were by 

 no means " small." The three curves correspond to three 

 separate adjustments of the centre of gravity of the beam, 

 the lower the centre of gravity the nearer the curve ap- 

 proaches to coincidence with the axes of h and S^. The 

 centres lie approximately on a straight line which cuts the 

 axis at a point corresponding to 3'4 turns from the initial 

 position of the screw. Thus 3'4 turns would bring P, fig. 1, 

 into the same vertical as Q. 



Next consider the effect of increasing the weight of a b, 

 fig. 1. It is clear from fig. 2 that the tendency will be" to 

 increase the sensitiveness, for a greater pull in the direction 



of increasing ^ will result. An increase of k 2 i — ) takes 



