Evening star. [volume] (Washington, D.C.) 1854-1972, 1926-01-07
FIFTEEN MINUTES OF
RADIO EACH DAY
BY gOSEPR CALCATERRA.
All Rights Reserved Repro. Prohibited.
The Theory and Practice Of Tuning
Part ILL.,


Part III.
In an ordinary direct-current circuit
there are three factors which deter
mine the conditions of current flow.
They are applied voltage. amount of
current flow or amperes and resist
ance in ohms.
Each is dependent upon the other,
and any variation of one causes a
variation OF the other factors. In
alternating current circuits the resist
ance in the circuit to the flow of a
current depends on other factors he
sides the actual ''ohmic' resistance
offered by materials which we ordi-
narlv think of "a offering resistance
to the flow of a current.
Current Flow Explained.
In a direct current circuit a coil Of
wire connected into the circuit resists
he now of current only to the extent
Of the resistivity of the wire OF which
the coil is made. In alternating cur
rent. especially in cases Of high-fre-
ouency alternating current. the &C
ton of the coil is quite different In
such cases the current is alternately
building up and collapsing, causing a
building up and collapsing Of the mag
netic lines of Aux which the pa ssage
Of current through a wire ca uses.
This produces an action in the cir
cult that H. similar in Its erect to the
introduction OF resistance.
The appare resistance caused by
the introduction of a coil or OF "A con
denser in an alternating current cir
cult Is called "reactance." That of
fered by a coil il called 'inductive
rea ctance" and that offered by a con
ienser is called Ca pacitive reactance.
The effects of inductance and capac-
ity occur when the voltage or current
in the circuit changes m intensity.
That is why they are obtained in an
alternating circuit where the voltage
and current is changing constantly.
Due to Change Of Current.
The effects due to inductance de-
pend on a change of current in the
circuit. The effects due to capacity
depend on a change in voltage in the
circuit. The reaction Of an inducta nee
in a circuit tends to prevent and
change in current while the reaction
of a condenser tends to prevent any
change in voltage.
The most important characteristic
of the action of inductance and capac-
ity in circuit is that the reaction
changes with the frequency. II is,
therefore, possible to have circuit
in which the reactance to a current
Of one frequency Is greater than the
reac ;ance to a current Of higher or
a lower frequency. BY properly ad-
just;ng the values of inductance and
cap; city. then. it is possible to adjust
a circuit to offer very little resistance
to the flow of a current of one fire.
auency, but high resistance to the
flow of current of other frequencies.
This Is the secret of tuning a radio
circuit The circuit is adjusted to
offer very little resistance to the flow
of a current Of the desired frequency
or wave length. In that condition
the circuit offers considerable resist
ance to the flow. of current of other
frequencies, so that the effect Ia to
cause the selection OF the desired FIRE.
quency and the rejection Of the
others.
The combined resistance offered by
the actual or direct current resistance
of the circuit. the inductive reactance
and capacitive reactance, is called the
'impedance" Of the circuit