Fig. 168. "FARADAY'S NET."
Faraday's Net.
An apparatus for showing that the electric charge resides on the
surface. It consists of a net, conical in shape and rather deep, to
whose apex two threads, one on each side, are attached. Its mouth is
fastened to a vertical ring and the whole is mounted on an insulating
support.
It is pulled out to its full extent and is electrified. No charge can be
detected inside it. By pulling one of the threads it is turned with the
other side out. Now all the charge is found on the outside just as
before, except that it is of course on the former inside surface of the
bag. The interior shows no charge.
Faraday's Transformer.
The first transformer. It was made by Michael Faraday. It was a ring of
soft iron 7/8 inch thick, and 6 inches in external diameter. It was
wound with bare wire, calico being used to prevent contact of the wire
with the ring and of the layers of wire with each other, while twine was
wound between the convolutions to prevent the wires from touching.
Seventy-two feet of copper wire, 1/20 inch diameter, were wound in three
superimposed coils, covering about one-half of the ring. On the other
half sixty feet of copper wire were wound in two superimposed coils.
Faraday connected his coils in different ways and used a galvanometer to
measure the current produced by making and breaking one of the circuits
used as a primary.
The coil is of historic interest.
Faraday's Voltameter.
A voltameter, in which the coulombs of current are measured by the
volume of the gas evolved from acidulated water. (See Voltameter, Gas.)
Faradic. adj.
Referring to induced currents, produced from induction coils. As Faraday
was the original investigator of the phenomena of electro-magnetic
induction, the secondary or induced electro-magnetic currents and their
phenomena and apparatus are often qualified by the adjective Faradic,
especially in electro-therapeutics. A series of alternating
electrostatic discharges, as from an influence machine (Holtz), are
sometimes called Franklinic currents. They are virtually Faradic, except
as regards their production.
251 STANDARD ELECTRICAL DICTIONARY.
Faradic Brush.
A brush for application of electricity to the person. It is connected as
one of the electrodes of an induction coil or magneto generator. For
bristles wire of nickel plated copper is generally employed.
Faradization.
In medical electricity the analogue of galvanization; the effects due to
secondary or induced currents; galvanization referring to currents from
a galvanic battery; also the process of application of such currents.
Faults.
Sources of loss of current or of increased resistance or other troubles
in electric circuits.
Feeder.
A lead in an electric central station distribution system, which lead
runs from the station to some point in the district to supply current.
It is not used for any side connections, but runs direct to the point
where current is required, thus "feeding" the district directly. In the
two wire system a feeder may be positive or negative; in the three wire
system there is also a neutral feeder. Often the term feeder includes
the group of two or of three parallel lines.
Feeder Equalizer.
An adjustable resistance connected in circuit with a feeder at the
central station. The object of the feeder being to maintain a definite
potential difference at its termination, the resistance has to be varied
according to the current it is called on to carry.
Feeder, Main or Standard.
The main feeder of a district. The standard regulation of pressure
(potential difference between leads) in the district is often determined
by the pressure at the end of the feeder.
Feeder, Negative.
The lead or wire in a set of feeders, which is connected to the negative
terminal of the generator.
Feeder, Neutral.
In the three wire system the neutral wire in a set of feeders. It is
often made of less diameter than the positive and negative leads.
Feeder, Positive.
The lead or wire in a set of feeders, which wire is connected to the
positive terminal of the generator.
Ferranti Effect.
An effect as yet not definitely explained, observed in the mains of the
Deptford, Eng., alternating current plant. It is observed that the
potential difference between the members of a pair of mains rises or
increases with the distance the place of trial is from the station.
[Transcriber's note: This effect is due to the voltage drop across the
line inductance (due to charging current) being in phase with the
sending end voltages. Both capacitance and inductance are responsible
for producing this phenomenon. The effect is more pronounced in
underground cables and with very light loads.]
252 STANDARD ELECTRICAL DICTIONARY.
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.
253 STANDARD ELECTRICAL DICTIONARY.
The distorted lines must be regarded as resultants of the two induced
polarities of the armature, one polarity due to the induction of the
field, the other to the induction from its own windings. The positions
of the brushes have much to do with determining the amount and degree of
distortion. In the case of the ring armature it will be seen that some
of the lines of force within the armature persist in their polarity and
direction, almost as induced by the armature windings alone, and leak
across without contributing their quota to the field. Two such lines are
shown in dotted lines.
In motors there is a similar but a reversed distortion.
Fig. 169. DISTORTION OF FIELD IN A
RING ARMATURE OF AN ACTIVE DYNAMO.
Fig. 170. DISTORTION OF FIELD IN A
RING ARMATURE OF AN ACTIVE MOTOR.
254 STANDARD ELECTRICAL DICTIONARY.
Field, Drag of.
When a conductor is moved through a field so that a current is generated
in it, the field due to that current blends with the other field and
with its lines of force, distorting the field, thereby producing a drag
upon its own motion, because lines of force always tend to straighten
themselves, and the straightening would represent cessation of motion in
the conductor. This tendency to straightening therefore resists the
motion of the conductor and acts a drag upon it.
Field of Force.
The space in the neighborhood of an attracting or repelling mass or
system. Of electric fields of force there are two kinds, the
Electrostatic and the Magnetic Fields of Force, both of which may be
referred to. A field of force may be laid out as a collection of
elements termed Lines of Force, and this nomenclature is universally
adopted in electricity. The system of lines may be so constructed that
(a) the work done in passing from one equipotential surface to the next
is always the same; or (b) the lines of force are so laid out and
distributed that at a place in which unit force is exercised there is a
single line of force passing through the corresponding equipotential
surface in each unit of area of that surface. The latter is the
universal method in describing electric fields. It secures the following
advantages:--First: The potential at any point in the field of space
surrounding the attracting or repelling mass or masses is found by
determining on which imaginary equipotential surface that point lies.
Second: If unit length of a line of force cross n equipotential
surfaces, the mean force along that line along the course of that part
of it is equal to n units; for the difference of potential of the two
ends of that part of the line of force = n; it is also equal to F s (F
= force), because it represents numerically a certain amount of work;
but s = I, whence n = F. Third: The force at any part of the field
corresponds to the extent to which the lines of force are crowded
together; and thence it may be determined by the number of lines of
force which pass through a unit of area of the corresponding
equipotential surface, that area being so chosen as to comprise the
point in question. (Daniell.)
Field of Force, Electrostatic.
The field established by the attracting, repelling and stressing
influence of an electrostatically charged body. It is often termed an
Electrostatic Field. (See Field of Force.)
255 STANDARD ELECTRICAL DICTIONARY.
Field of Force of a Current.
A current establishes a field of force around itself, whose lines of
force form circles with their centres on the axis of the current. The
cut, Fig. 172, shows the relation of lines of force to current.
Fig. 171. EXPERIMENT SHOWING LINES OF FORCE
SURROUNDING AN ACTIVE CONDUCTOR.
Fig. 172. DIAGRAM OF FIELD OF FORCE
SURROUNDING AN ACTIVE CONDUCTOR.