Farad.
The practical unit of electric capacity; the capacity of a conductor
which can retain one coulomb of electricity at a potential of one volt.
The quantity of electricity charged upon a conducting surface raises its
potential; therefore a conductor of one farad capacity can hold two
coulombs at two volts potential, and three coulombs at three volts, and
so on. The electric capacity of a conductor, therefore, is relative
compared to others as regards its charge, for the latter may be as great
as compatible with absence of sparking and disruptive discharge. In
other words, a one farad or two farad conductor may hold a great many
coulombs. Charging a conductor with electricity is comparable to pumping
air into a receiver. Such a vessel may hold one cubic foot of air at
atmospheric pressure and two at two atmospheres, and yet be of one cubic
foot capacity however much air is pumped into it.
The farad is equal to one fundamental electrostatic unit of capacity
multiplied by 9E11 and to one electro-magnetic unit multiplied by 1E-9.
The farad although one of the practical units is far too large, so the
micro-farad is used in its place. The capacity of a sphere the size of
the earth is only .000636 of a farad.
[Transcriber's note: Contemporary calculations give about .000720
farad.]
Faraday, Effect.
The effect of rotation of its plane produced upon a polarized beam of
light by passage through a magnetic field. (See Magnetic Rotary
Polarization.)
Faraday's Cube.
To determine the surface action of a charge, Faraday constructed a room,
twelve feet cube, insulated, and lined with tinfoil. This room he
charged to a high potential, but within it he could detect no excitement
whatever. The reason was because the electricity induced in the bodies
within the room was exactly equal to the charge of the room-surface, and
was bound exactly by it. The room is termed Faraday's cube.
Faraday's Dark Space.
A non-luminous space between the negative and positive glows, produced
in an incompletely exhausted tube through which a static discharge, as
from an induction coil, is produced. It is perceptible in a rarefaction
of 6 millimeters (.24 inch) and upwards. If the exhaustion is very high
a dark space appears between the negative electrode and its discharge.
This is known as Crookes' dark space.
Faraday's Disc.
A disc of any metal, mounted so as to be susceptible of rotation in a
magnetic field of force, with its axis parallel to the general direction
of the lines of force. A spring bears against its periphery and another
spring against its axle. When rotated, if the springs are connected by a
conductor, a current is established through the circuit including the
disc and conductor. The radius of the disc between the spring contacts
represents a conductor cutting lines of force and generating a potential
difference, producing a current. If a current is sent through the
motionless wheel from centre to periphery it rotates, illustrating the
doctrine of reversibility. As a motor it is called Barlow's or
Sturgeon's Wheel. If the disc without connections is rapidly rotated it
produces Foucault currents, q. v., within its mass, which resist its
rotation and heat the disc.