(1) In an ac circuit, the

current

continually

changes

amplitude

and

periodically

reverses

direction.

Any

such

change causes an expansion

or

contraction

of

the

magnetic field. The lines

of force in this changing

field cut the turns of

wire in the coil, inducing

a voltage which opposes

the

applied

ac

voltage

that

is

pushing

the

current

through

the

windings of the coil. The

property of a coil to

develop

an

opposing

voltage is known as self-

inductance.

(2) Direct

application

of

self-inductance

can

be

seen in a radio-frequency

(RF) choke coil. The five

smaller coils in A of

figure 18 are examples of

RF choke coils.

Their

function is to oppose the

flow of RF current, but at

the same time allow the

passage of dc.

They are

widely

used

in

radio

b. Effect of Inductance.

The

magnitude (amount) of counter EMF

produced by the self-inductance of a

coil depends on two factors: the

number of turns in the coil and the

frequency of the applied voltage.

(1) The number of turns in the

coil

determines

the

physical

property

of

inductance.

The greater

the number of turns, the

Figure 21.

Representative capacitor

larger the inductance of

types.

the coil. And, of course,

the larger the inductance,

the more counter EMF is

produced.

(2) Since the counter EMF voltage produced by the self-inductance of a coil

depends upon the rate at which the lines of force cut the turns, it

follows that the higher the cutting rate, the greater will be the