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Reader Feedback (04.2020)

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I Spy

I enjoyed the recent spy article in Nuts & Volts. A nice mix of hardware and an interesting use. It took me a couple of minutes to get my head around the plain text table, but once I did I now have the all important message! I shall now burn my copy of Nuts & Volts so it doesn’t fall into the wrong hands!

Bob Liesen WB0POQ

Not So Faraday’s Law

Previously, I wrote to Bryan Bergeron on this subject and have had some more insights. I don’t believe Faraday’s Law of Electromagnetism belongs in textbooks.

It is false, serves no useful purpose, only serves to mislead people, and is not even physics. I’m referring to the version of Faraday’s Law which uses the ordinary time derivative and not the Maxwell-Faraday Law (one of Maxwell’s equations) which uses the partial time derivative and is valid.

Richard Feynman pointed out in The Feynman Lectures on Physics that Faraday’s Law does not work for the homopolar generator. Ironically, my college physics textbook (Sears and Zemansky) erroneously uses the homopolar generator as an example of the application of Faraday’s Law and gives a spurious analysis. Since the homopolar generator is a counter-example to Faraday’s Law, this proves that the law is false.

Faraday’s Law utilizes the time derivative of the magnetic flux linking the circuit regardless of whether it’s due to the motion of the circuit or part of the circuit, or due to an intrinsic changing of the magnetic field. Each of the two cases depends on a different one of the two principles of induction, motional EMF, and the Maxwell-Faraday Law.

In the case of a loop of wire moving through a non-uniform magnetic field, determining the value of the motional EMF and determining the EMF by Faraday’s Law involves exactly the same calculations. Therefore, Faraday’s Law serves no useful purpose and could be eliminated without being missed.

Using the time derivative of the magnetic flux linking the circuit to determine the EMF of a moving circuit is misleading because the cause is obscured. The EMF results from the motion of the circuit through the magnetic field, and the changing linkage of the circuit is irrelevant. It’s a case of correlation without causation.

Faraday’s Law depends on a mathematical coincidence. It does not express a physical principle. It is solely math and not physics.

Michael S. La Moreaux

Miss Q

While the article, Oscillators, Statistics, Q, and Muons was informative, there were two errors. The first was about the range of the one standard deviation. If the SD is 0.2, then the range is 0.978 to 1.018. The article had 0.996 to 1.018. A simple math error.

The other problem is more troubling. The last paragraph under the heading “The Quality (or Q) of an Oscillator” makes no sense. The author was talking periods throughout the article and switches to frequency.

A 1 MHz oscillator has a period of 1 µsec. If it has the same relative spread (0.047 µsec), then Q is the same. If it has a spread of 0.047 seconds, then Q is 2.1E-5 — a terrible quality factor. I can not see any reading that would come to the conclusion stated.

Leave out everything but the first sentence of the paragraph and it makes sense.

William Massano

I do have a couple typos in my article on “Oscillators, Statistics, Q, and Muons.” Thank you to this diligent reader for pointing them out. 1) If a pendulum has an average swing time of 0.998 seconds with a standard deviation of 0.02 seconds, then 68% of the time, it’ll swing with a period of between 0.978 and 1.018 s (not 0.996 s as stated).

2) When discussing the hypothetical 1 MHz oscillator, it would be an incredible oscillator (with a very high Q) because its f (frequency) is so large compared with 0.047 Hz (not its T as stated). Sorry, the T should have been an f.

I apologize for these typos (I should have caught the T vs. f typo since MHz is a unit of frequency and not period); the blame is all on me. I hope this helps.

Tom Bensky

NOS is Best

I agree with Bryan Bergeron about much of what he mentioned in his recent editorial about NOS, particularly resistors, capacitors, and selenium rectifiers. However, in some cases, NOS is a must.

Some tubes are just not available and NOS or used must be obtained to run the radio. More importantly, he mentioned that panel lamps can be replaced by LEDs but, again, in some cases, use of LEDs will shorten the life of rectifier tubes like 35Z5s. Those tubes were designed so that a filament tap is brought out for an incandescent pilot lamp of 6-7 volts and is often connected to the plate of that tube as well.

The lamp acts as a shunt and if it burns out, the full surge of current goes through the filament when the radio is turned on and even after warm-up, the current is higher than designed. A 300 ohm resistor or higher shunted across the tap will protect the tube if LEDs are used. As I understand it, if the radio uses one of these types of tubes and the lamp is not functioning, it’s recommended to turn off the radio and replace it immediately.

Mike Goldberg

Thanks for the feedback. This is the best way for readers — including myself.

I agree with you, re: NOS. In the radios that I’ve featured (there’s more to come), the lamps aren’t used for anything other than illumination. However, as you correctly point out, that isn’t always the case. In some cases, the filaments — tubes and bulbs — are wired in series, and simply replacing a bulb with an LED won’t do. Then there’s the case you point out.

I’ve made a point of avoiding such circuits thus far, but perhaps it would be informative for me to dig a bit deeper back in time and illustrate at least one of these circuits during a rebuild.

Thanks again for taking the time to write.

Bryan Bergeron

Tube Dude

I have really enjoyed Bryan Bergeron’s editorials and columns of late on tube devices. I hope his basic well-written discussions will attract more folks to this exciting part of our hobby. I seem to always have at least one tube device on the bench!

It was the glow of tubes and the warmth of both the sound and the radio itself that got me interested in radio back in the ‘60s — a Zenith Transoceanic (which I still have). In recent years, I’ve restored a Hallicrafters SX-111, a Capehart T30 (classic All American 5 from 1951), and a couple of other table tops.

Keep up the good work.

Trent Fleming N4DTF

I’m glad you’re enjoying the magazine. I think our shacks must look alike. Tube devices everywhere. Running out of space ...

My plan is to have one tube article in every issue. There’s an upcoming DX-60B article, as well as a Drake 2B. Plus a few more German receivers.

Bryan Bergeron