94 
Proceedings of Boyal Society of Edinburgh. [sess. 
Magnetic Shielding in Hollow Iron Cylinders. 
By James Russell. 
(Read February 3, 1902.) 
{Abstract.) 
This investigation deals with the shielding which exists within 
hollow iron cylinders when placed in a uniform transverse magnetic 
field. This transverse field divided by the internal magnetic field 
is defined as the shielding ratio. For the thin iron cylinders ex- 
perimented with, it appears from various mathematical contributions 
to the subject that the shielding ratio minus unity may for present 
purposes be taken to be proportional to certain geometrical data 
and to the permeability (the permeability being large). By means 
of a rotating inductor within the shield, connected with a ballistic 
galvanometer, experimental determinations of the shielding ratio 
are made under various conditions of magnetisation ; and an 
endeavour is also made to show how far and under what conditions 
these results approximate to theoretical formulae which assume 
the permeability to be uniform all round the shield, and the 
absence of retentivity and coercive force in the iron. Two iron 
shields were experimented with, the hysteretic constants {rj) being 
•0015 and *0028 respectively. The following results have been 
arrived at, viz. : — 
I. When no other magnetising force is acting upon the iron 
than that due to the transverse field increased by increments from 
zero, the shielding ratio minus unity is proportional to what may 
be called the ratio permeability (B/H), and not to the differential 
permeability (dB/dR). If, however, the transverse field is de- 
creased by steps from a maximum, the theoretical conditions are 
not fulfilled, and the shielding ratio increases very rapidly and 
becomes infinite, but does not change sign if account be taken of 
the negative residual field within the shield when at each step 
the transverse field is withdrawn. 
II. When a circular magnetising force is acting upon the iron 
