Magnetic Shield.
In general a magnetic screen, q. v. Sometimes a strong local field is
made to act as a shield, by its predominance overcoming any local or
terrestrial field to which the needle to be protected may be exposed.
Magnetic Shunt.
The conception of a magnetic circuit being formed, the shunt is a
corollary of the theory. It is any piece of iron which connects points
of a magnet differing in polarity, so as to divert part of the lines of
force from the armature or yoke. The shunt is especially applicable in
the case of horseshoe magnets. Thus a bar of iron placed across from
limb to limb a short distance back from the poles would act as a shunt
to the armature and would divert to itself part of the lines of force
which would otherwise go through the armature and would weaken the
attraction of the magnet for the latter. In dynamos a bar of iron used
as a magnetic shunt has been used to diminish the lines of force going
through the armature and hence to weaken the field and diminish the
electro-motive force. By moving the shunt nearer or further from the
poles the dynamo is regulated.
In the cut the projections between the yoke and poles of the magnet
shown act as a shunt to the yoke, taking some lines of force therefrom.
Fig. 225. MAGNETIC SHUNT.
Magnetic Storms.
Terrestrial magnetic disturbances sometimes covering very wide areas,
and affecting the magnetic declination and inclination. One such
disturbance was felt simultaneously at Toronto, Canada, the Cape of Good
Hope, Prague and Van Diemen's Land. (Sabine.)