Figure 22.

Schematic symbols for capacitors.

counter EMF produced.

The rate of change of the magnetic flux is

controlled by the frequency of the applied voltage. A coil intended to

be used at high frequencies can therefore be smaller than one intended

for low frequencies for the same amount of opposition (counter EMF) to

current flow. The difference in size of the coils will be evident if

you have occasion to compare the RF inductors of a television receiver

with those in a broadcast radio receiver.

(3) In the foregoing discussions, the opposition offered to a specific

change of ac by an inductance was measured at any given instant in

terms of counter EMF, the voltage opposing the applied voltage. In dc

circuits, however, any opposition to current flow is termed resistance

and is measured in ohms. In ac circuits, therefore, it is convenient

to measure inductive opposition in ohms rather than in volts of counter

EMF. This type of ac opposition is called inductive reactance and is

assigned the symbol XL to distinguish it from dc resistance, R. From

here on, we will refer to the opposition offered by an inductance to ac

flow as inductive reactance XL.

(4) The unit of measure of inductance is the henry (h). Since small values

of inductance are used in radio circuits, it is customary to rate the

inductance value in millihenries or microhenries.

(a) 1 millihenry = 1/1,000 henry = 0.001 henry = 1 x 10-3 henry.

(b) 1 microhenry = 1/1,000,000 henry = 0.000,001 henry = 1 x 10-6 henry.

(5) Current through an inductance does not increase in proportion to the

change of voltage across the inductance. This is due to the fact that

a time delay exists in the buildup of the magnetic field to the maximum

value. To explain this delay time, it is customary to say