Ferro-magnetic. adj.
Paramagnetic; possessing the magnetic polarity of iron.
Fibre and Spring Suspension.
A suspension of the galvanometer needle used in marine galvanometers.
The needle is supported at its centre of gravity by a vertically
stretched fibre attached at both its ends, but with a spring
intercalated between the needle and one section of the fibre.
Fibre Suspension.
Suspension, as of a galvanometer needle, by a vertical or hanging fibre
of silk or cocoon fibre, or a quartz fibre. (See Quartz.)
This suspension, while the most delicate and reliable known, is very
subject to disturbance and exacts accurate levelling of the instrument.
Fibre suspension is always characterized by a restitutive force. Pivot
suspension, q. v., on the other hand, has no such force.
Field, Air.
A field the lines of force of which pass through air; the position of a
field comprised within a volume of air.
Field, Alternating.
Polarity or direction being attributed to lines of force, if such
polarity is rapidly reversed, an alternating field results. Such field
may be of any kind, electro-magnetic or electrostatic. In one instance
the latter is of interest. It is supposed to be produced by high
frequency discharges of the secondary of an induction coil, existing in
the vicinity of the discharging terminals.
Field Density.
Field density or density of field is expressed in lines of force per
unit area of cross-section perpendicular to the lines of force.
Field, Distortion of.
The lines of force reaching from pole to pole of an excited field magnet
of a dynamo are normally symmetrical with respect to some axis and often
with respect to several. They go across from pole to pole, sometimes
bent out of their course by the armature core, but still symmetrical.
The presence of a mass of iron in the space between the pole pieces
concentrates the lines of force, but does not destroy the symmetry of
the field.
When the armature of the dynamo is rotated the field becomes distorted,
and the lines of force are bent out of their natural shape. The new
directions of the lines of force are a resultant of the lines of force
of the armature proper and of the field magnet. For when the dynamo is
started the armature itself becomes a magnet, and plays its part in
forming the field. Owing to the lead of the brushes the polarity of the
armature is not symmetrical with that of the field magnets. Hence the
compound field shows distortion. In the cut is shown diagrammatically
the distortion of field in a dynamo with a ring armature. The arrow
denotes the direction of rotation, and n n * * * and s s * * * indicate
points of north and south polarity respectively.