September 1961 Popular Electronics
Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954  April 1985. All copyrights are hereby acknowledged.

Have you ever heard of a "swinging choke?" I surely hadn't,
so my probability of getting Inductance Quiz question number
5 correct was 50% at best. I guessed wrong  just my luck. As
a result my score was 8/9 = 89%. Oh, the shame. Maybe you will
have a better time of it. Be careful with Q6 as well. Otherwise,
if you understand the fundamentals of inductor circuit analysis,
you will have no problem.
Inductance Quiz
Inductance, as you may know, is the electrical property frequently
compared to mechanical inertia. To gauge your "inductance" knowledge,
solve the problems below, then check your answers.
By Robert P. Balin
1  The larger the resistance, the greater
the voltage developed on opening the switch.
True
False
2  Current will continue to flow, even
after the supply voltage has dropped to zero.
True
False
3  Increasing the supply frequency will
cause the lamp to glow more brightly.
True
False
4  Bunching a number of turns together
in a coil will increase its inductance.
True
False
5  The inductance of a "swinging" choke
decreases as the current through it increases.
True
False
6  Inserting a brasstipped tuning wand into
a coil will increase its inductance.
True
False
7  The lamp will glow more brightly as
the iron core is moved out of the coil.
True
False
8  Since a bifilar winding is "doubled
back" on itself, it boosts inductance.
True
False
9  The tuning slug on an oscillator coil
is most withdrawn at the top end of the band.
True
False
Answers
1  True. When
the switch is opened, the inductance of the coil tends to maintain
the same value of current flow in the circuit. And the higher
the value of the series resistance, the greater the e.m.f. which
will be developed.
2  True. Since
current lags voltage by 90 degrees in a purely inductive circuit,
current will continue to flow after the voltage has dropped
to zero.
3  False. Because
of the back e.m.f. induced in the coil as the current through
it changes, the greater the rate of current change, the greater
is the opposition to such change. Thus, the higher the frequency
of the current through the coil, the greater the inductance,
and the smaller the voltage delivered to the lamp.
4  True. Closely
spacing a number of turns in a coil will increase the strength
of its magnetic field and thus its inductance.
5  True. A swinging
choke is an inductor which is designed to reach a maximum amount
of magnetization or "saturation" at low values of rated current.
From this point on, an increase in the amount of current reduces
the degree of magnetization and hence the inductance. A greater
portion of the source voltage therefore becomes available to
compensate for the larger resistive voltage drops occurring
within the power supply.
6  False. Eddy
currents induced in the brass will produce a magnetic field
which opposes that of the coil and thus effectively reduces
the coil's inductance.
7  True. The
iron core serves to increase the coil's inductance, leaving
only a small voltage available to light the lamp. Removing the
core therefore increases the voltage applied to the lamp.
8  False. The
current in this type of coil flows in opposite directions in
adjacent turns. Back e.m.f.'s of selfinduction are produced
in all of the turns; but since the back e.m.f.'s of mutual induction
will all have the opposite polarity, they cancel out the back
e.m.f.'s of selfinduction and thus make the coil "noninductive."
9  True. When
the slug is moved out of the coil, the "core" consists of air,
and the inductance of the coil is decreased. Since the smaller
magnetic field is able to expand and contract at a faster rate,
the coil is now able to transfer its electrical energy into
the resonating capacitor at a higher frequency.
Posted May 15, 2014