1901 - 2 .] Mr James Bussell on Magnetic Shielding. 95 
in addition to that due to the transverse field, the order and manner 
in which the one field is superposed upon the other affects the 
shielding ratio to an enormous extent. The conclusions arrived at 
are not in harmony with the inductions drawn from the investi- 
gations of Stefan and Du Bois, which “lend support to the 
assumption made throughout that shielding against moderate dis- 
turbing fields depends within due limits upon the constant permea- 
bility for small increments or decrements of force superimposed 
upon any condition of magnetisation.” (See Electrician , “ Magnetic 
Shielding,” Du Bois, vol. 40, p. 654, 1898.) 
It is found that when upon a pre-existing induction due to the 
transverse field, increments of the circular field are superposed (T C 
conditions), the shielding ratio minus unity is proportional to the 
differential permeability as impressed upon the iron by the circular 
field. The shielding ratios plotted as ordinates against the corre- 
sponding values of the circular field as abscissae rapidly increase, 
attain a maximum, and then decrease, finally approximating to a 
minimum asymptotic value. If, however, upon a pre-existing in- 
duction, due to the circular field increasing from zero, the transverse 
field is superposed at each increment (C T conditions), the 
shielding ratio curves take much lower values. The superposition 
of repeated reversals of the transverse field (C T T conditions) still 
further lowers the shielding ratio, and in this case there appears to 
be no initial rise as the curve leaves its origin in the vertical axis. 
On the other hand, the superposition of repeated reversals of the 
circular field (T C C conditions) increases the shielding ratio so 
long as the values of (dB/dU.) are high. When the values of 
dB/dK are a maximum, the shielding ratio under the T C C con- 
ditions are six to seven times greater than under the C T T con- 
ditions. The curves obtained under all the conditions of super- 
position of fields appear to approximate to the same minimum 
value as the circular magnetising force is further and further 
increased. 
A distinction is drawn between the permeability of the iron to 
the circular field and the permeability of the iron to the trans- 
verse field. Thus, while under the T C conditions the shielding 
ratio minus unity is proportional to the permeability ( dB/dR ) of 
the iron to the circular force, the same does not hold good under, 
