Inductance
When a current flows through a conductor it sets up a magnetic field in the
neighbourhood of the conductor. This is negligible in its effects in a number of cases, but
there are also many cases where this magnetic field exerts a profound effect
upon the circuit.
The magnetic field created by the current is representend by lines of magnetic flux, these
lines consisting of closed loops which are interlinked with the electric circuit, itself
necessarily a closed circuit. If the current is steady, t
he magnetic flux is constant and
produces no effect upon the
circuit, but if the current changes then the strength of the
magnetic field also changes. If the current increases, the total number of lines of magnetic
flux is increased, so that the total numb
er of flux

linkages is also increased. It is, however, a
fundamental law that whenever the number of flux

linkages changes, an e.m.f. is induced in
the circuit linked with the flux. This e.m.f. is proportional to the rate of change of linkages,
and one vol
t is induced when the linkages change at the rate of 10
8
linkages per second.
Unit of Inductance.
—
The unit of inductance is the henry, and a circuit is said to
possess an inductance of 1 henry if 1 volt is induced when the current changes at the rate of
1 ampere per second. The symbol for inductance is L, so that a circuit possesses an
inductance of L henries if L volts are induced due to a rate of change of current of 1 ampere
per second.
This induced e.m.f. always acts in such a direction as to oppose t
he change of current in
the circuit, and also the magnetic flux linked with it. Thus, if the current is rising, inductance
tends to oppose its growth, and if the current is falling inductance tends to oppose its decay.
Examples of this effect are found in
the field circuit of an ordinary generator or motor, the
field circuit being highly inductive. When switched into circuit the current does not
immediately rise to its full value, but grows relatively slowly, while when the switch is
opened, the current ten
ds to continue as evinced by the spark at the opening contacts.
Mutual Inductance.
—
When inductance is due to flux linking with the same circuit that
carries the current, it is called self

inductance, in order to distinguish it from inductance due
to flux l
inking with the circuit due to current in a neighbouring" circuit, which is called
mutual inductance. In the latter case, two circuits are said to possess mutual inductance if a
change of current in one circuit causes an e.m.f. to be induced in the other c
ircuit.
Two circuits are said to possess a mutual inductance of L henries if L volts are induced
in one circuit due to a rate of change of current of 1 ampere per second in the other circuit.
The circuit in which the current is changing is called the prima
ry circuit, while the circuit in
which the e.m.f. is induced is called the secondary circuit. When two circuits possess mutual
inductance, either circuit can be employed as the primary, the value of the mutual inductance
being the same, no matter which cir
cuit is the primary and which
the secondary is
.
Importance of Inductance in A. C. Circuits.
—
Inductance is a property of a circuit,
just as is resistance, and is therefore possessed by d.c. as well as a.c. circuits. In d.c. circuits,
however, its effects
are not apparent when the current is steady, and are only noticeable
when the current is started or stopped, or when it changes in value. The reason for this is that
the induced e.m.f. resulting from inductance is due to the rate at which the current, and
therefore the flux is changing. If the current does not change, there is no induced e.m.f. In
a.c. circuits, on the other hand, the current is always changing,' and therefore the e.m.f. of
self

induction is always present. This modifies the value of the cu
rrent and has far

reaching
effects.
М.А. Беляева и др. «Сборник технических текстов на англ. Языке»
Inductance
When a current flows through a conductor it sets up a magnetic field in the
neighbourhood of the conductor. This is negligible in its effects in a number of cases, but
there are al
so many cases where this magnetic field exerts a profound effect upon the circuit.
The magnetic field created by the current is representend by lines of magnetic flux, these
lines consisting of closed loops which are interlinked with the electric circuit,
itself
necessarily a closed circuit. If the current is steady, the magnetic flux is constant and
produces no effect upon the circuit, but if the current changes then the strength of the
magnetic field also changes. If the current increases, the total numbe
r of lines of magnetic
flux is increased, so that the total number of flux

linkages is also increased. It is, however, a
fundamental law that whenever the number of flux

linkages changes, an e.m.f. is induced in
the circuit linked with the flux. This e.m.f
. is proportional to the rate of change of linkages,
and one volt is induced when the linkages change at the rate of 10
8
linkages per second.
Unit of Inductance.
—
The unit of inductance is the henry, and a circuit is said to
possess an inductance of 1 hen
ry if 1 volt is induced when the current changes at the rate of
1 ampere per second. The symbol for inductance is L, so that a circuit possesses an
inductance of L henries if L volts are induced due to a rate of change of current of 1 ampere
per second.
Th
is induced e.m.f. always acts in such a direction as to oppose the change of current in
the circuit, and also the magnetic flux linked with it. Thus, if the current is rising, inductance
tends to oppose its growth, and if the current is falling inductance
tends to oppose its decay.
Examples of this effect are found in the field circuit of an ordinary generator or motor, the
field circuit being highly inductive. When switched into circuit the current does not
immediately rise to its full value, but grows rel
atively slowly, while when the switch is
opened, the current tends to continue as evinced by the spark at the opening contacts.
Mutual Inductance.
—
When inductance is due to flux linking with the same circuit that
carries the current, it is called self

indu
ctance, in order to distinguish it from inductance due
to flux linking with the circuit due to current in a neighbouring" circuit, which is called
mutual inductance. In the latter case, two circuits are said to possess mutual inductance if a
change of curr
ent in one circuit causes an e.m.f. to be induced in the other circuit.
Two circuits are said to possess a mutual inductance of L henries if L volts are induced
in one circuit due to a rate of change of current of 1 ampere per second in the other circuit.
The circuit in which the current is changing is called the primary circuit, while the circuit in
which the e.m.f. is induced is called the secondary circuit. When two circuits possess mutual
inductance, either circuit can be employed as the primary, the va
lue of the mutual inductance
being the same, no matter which circuit is the primary and which the secondary is.
Importance of Inductance in A. C. Circuits.
—
Inductance is a property of a circuit,
just as is resistance, and is therefore possessed by d.c. a
s well as a.c. circuits. In d.c. circuits,
however, its effects are not apparent when the current is steady, and are only noticeable
when the current is started or stopped, or when it changes in value. The reason for this is that
the induced e.m.f. resulti
ng from inductance is due to the rate at which the current, and
therefore the flux is changing. If the current does not change, there is no induced e.m.f. In
a.c. circuits, on the other hand, the current is always chang
ing,' and therefore the e.m.f. of
self

induction is always present. This modifies the value of the current and has far

reaching
effects.
М.А. Беляева и др. «Сборник технических текстов на англ. Языке»
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