Fly or Flyer, Electric.
A little wheel, ordinarily poised on a point, like a compass needle. It
carries several tangentially directed points, all pointing in the same
sense. When connected with a source of electricity of high potential it
revolves by reaction. The tension of its charge is highest at the
points, the air there is highly electrified and repelled, the reaction
pushing the wheel around like a Barker's mill or Hero's steam engine.
Sometimes the flyer is mounted with its axis horizontal and across the
rails on a railroad along which it travels.
Synonym--Reaction Wheel.
Foci Magnetic.
The two points on the earth's surface where the magnetic intensity is
greatest. They nearly coincide in position with the magnetic poles.
Fog, Electric.
Fogs occurring when the atmosphere is at unusually high potential and
accompanied by frequent change of such polarity.
Following Horns.
In dynamo-electric machines the projecting ends of the pole pieces
towards which the outer uncovered perimeter of the armature turns in its
regular operations. The leading horns are those away from which the
armature rotates. In considering rotation the exposed portion of the
superficies of the armature is considered. The definition would have to
be reversed if the part facing the pole pieces were considered.
Synonym--Trailing Horns.
Foot-candle.
A unit of illuminating power; the light given by one standard candle at
a distance of one foot. The ordinary units of illuminating power are
entirely relative; this is definite. It is due to Carl Herring.
Foot-pound.
A practical unit of work or energy. The quantity of work required to
raise a pound one foot, or one hundred pounds one-hundredth of a foot,
and so on; or the potential energy represented by a weight at an
elevation under these conditions.
Foot-step.
In a dynamo with armature at the lower end of its field magnets, the
plate generally of zinc, interposed between it and the iron base plate
to prevent the leakage of lines of force outside of the circuit. Any
diamagnetic material which is mechanically suitable may be used.
Force.
Force may be variously defined.
(a) Any cause of change of the condition of matter with respect to
motion or rest.
(b) A measurable action upon a body under which the state of rest of
that body, or its state of uniform motion in a straight line, suffers
change.
(c) It may be defined by its measurement as the rate of change of
momentum, or
(d) as the rate at which work is done per unit of space traversed.
Force is measured by the acceleration or change of motion it can impart
to a body of unit mass in a unit of time, or, calling
force, F,
mass, m
acceleration per second a
we have F = m a.
The dimensions of force are
mass (M) * acceleration (L/(T^2)) = (M*L)/(T^2).