724 
PROFESSOR F. G. BAILY ON THE HYSTERESIS OF 
Test of Permeability and Hysteresis in a Reversing Field. 
The method adopted was that used by Ewing in his more recent experiments and 
described by him in his paper ( £ Phil. Trans.,’ 1894), in which a continuous ring of iron 
is wound uniformly round with a magnetising coil, and a secondary coil is connected 
to a ballistic galvanometer. The current is always brought to the maximum value, 
and the throw of the galvanometer taken when the current is reduced (counting 
the sign algebraically), until the value of the reversed initial current is reached. 
Each change being practically instantaneous, it is probable that this method gives 
much the same value for the hysteresis as a rapidly alternating current does, but 
the point has not been experimentally proved. Dr. ,Iohn Hopkinson’s tests* on 
hardened steel show close similarity in the permeability with an alternating current 
and with a step-by-step method, but his experiments are not conclusive for the 
hysteresis value. As the iron was well laminated, there would be, according to 
Ewing, very little creeping effect in the magnetisation of the ring. For this purpose 
rings were made of the iron and steel, consisting of a narrow strip wound in several 
layers, the samples being taken from portions of the sheets of material used for 
making the armatures, and the physical conditions being kept as nearly as possible 
unchanged. Primary and secondary windings were wound on, the secondary being 
of fine wire placed immediately on the iron, so that no correction for air space was 
necessary. The dimensions of the rings were :— 
Soft iron. —Area of metal.= ’0730 sq.- centim. 
Mean circumference of ring = 20‘5 centims. 
Steel. —Area of metal.= *0366 sq. centim. 
Mean circumference . . . = 22‘8 centims. 
The method being well known requires no further description. 
The hysteresis is given by the expression ^ jlHdB, where JHdB is the area of the 
curve. B and H being in absolute measure, the hysteresis is expressed in ergs per 
cubic centimetre per cycle. 
By measuring the areas of the closed curves and dividing by 4v the following 
values were obtained :— 
Soft Iron.— B = 3,060 Hysteresis = 1,280 ergs. 
6,850 4,140 „ 
10,300 7,450 ,. 
11,800 9,100 „ 
14,100 12,600 „ 
* ‘Electrician,’ September, 1892, and May, 1893, 
