Inductance Quiz
September 1961 Popular Electronics

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

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 brass-tipped 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9 - The tuning slug on an oscillator coil is most withdrawn at the top end of the band.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

See answers below.

Popular Electronics published many quizzes over the years (as did a few other magazines to a lesser extent) - some really simple and others not so simple. Robert P. Balin created many of the quizzes. This is a listing of all I have posted thus far.

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 self-induction 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 self-induction and thus make the coil "non-inductive."

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