the Magnetic Properties of Iron. 



253 



Correction for Demagnetising Action of Poles. 



The correction for the demagnetising action due to the flux 

 produced is proportional to the total flux, and was determined by 

 obtaining the BH curves for samples consisting of 2, 4, 6, and 8 

 similar iron wires ; these were plotted with readings of the 

 H scale for abscissae, and readings of the B scale, divided by the 

 number of wires in sample, for ordinates ; it was then seen that 

 by shearing back the axis corresponding to H = zero by an amount 

 equal to the B scale reading, the curves became nearly coincident, 

 and consequently if this axis is always sheared back by this 

 amount, the resulting curve referred to rectangular coordinates is 

 corrected for the demagnetising action. 



Fig. 4 shows the curves thus plotted for the samples consisting 

 of 4 and 8 wires respectively. 



v 12 6\ 1'5| B> 



Value of Scale Divisions. 



The approximate value of the scale divisions in c.G.s. units 

 was determined by obtaining first of all the controlling or restoring 

 couple for a deflection of one division, and then with a sample of 

 known maximum permeability in the instrument a BH curve was 

 obtained, these data, together with the dimensions of the sample, 

 being sufficient for the purpose. 



The controlling couple was determined both by direct observa- 

 tion of the deflection produced by a known weight suspended at 

 a measured distance from the axis, and also by observing the 

 periodic time of the cradle portion for small oscillations, firstly 

 under normal adjustment and then with the addition of a brass 



