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Next > The More Things Change...
"The more things change, the more they stay the same." The adage is true in a
general sense for most things, including electricity and electronics. Sometimes,
though, another adage, "Leave well-enough alone," is ignored, resulting in a lot
of unnecessary confusion. I have lately run across examples that prove both to be
true.
You might have noticed in the last couple months the addition to RF Cafe of many
pages of chapters from a series of basic electricity and electronics course books
that were given to me by my father-in-law, Marlet Goodwin. He was in the Merchant
Marines and then the Naval Reserve for the few years around the end of World War
II. Two of those books, "Electricity - Basic Navy Training Courses (NAVPERS 10622)," and
"Electricians Mate 3 - Navy Training Courses (NAVPERS 10548)," have proven to be
an incredibly complete and well-organized collection of instruction. Since RF Cafe
is frequented by all manners of people interested in electronics and electricity
- engineers, managers, college and high school students, hobbyists, and parents
seeking help for their kids - posting them here seemed like the right thing to do.
In order to maximize the content's usefulness, I went to the trouble of scanning
each page and then using an OCR (optical character recognition) program to convert
the page image to actual searchable text, and then touched up all the figures using
a graphics editing program. It was a lot of work, but well worth the effort. There
is still a lot remaining to be posted.
Not wanting to be guilty of violating any copyrights, I consulted the
CENDI * website's "Frequently Asked Questions About Copyright Issues
Affecting the U.S. Government CENDI/2004-8 Updated March 2007." Basically, it states
that works created by the government are in the public domain, and may be replicated
(I am diligent in providing proper credit on all pages).
Finding as complete of an introduction to the fundamentals of electricity and
magnetism on the Internet for FREE is very difficult. Most of what I have found
is either nowhere near as comprehensive, or is available only for purchase. The
Navy manuals writers start with an assumption of no specific knowledge, but an ability
to learn quickly on the part of the student. There are even quizzes with answers
at the end of each chapter.
So, the point of my cogitation is that even though these courses were written
sixty years ago, the material is still relevant. Ohm's Law applied in the 1940s
just as it applies today. Chapters on inductive and capacitive reactance use the
same formulas that I used in my electronics courses, series and parallel circuits
behave like today's, transformer mutual inductance and counter electromotive force
(EMF) still works the same way, AC and DC motors rotate according to the familiar
rules. Batteries and cables, although constructed with different methods and materials
now, are fundamentally the same. Even the chapter on vacuum tubes still applies...
assuming you still happen to use tube systems (Hams should love that chapter).
There is one difficulty, however, that turned up in the electromagnetic induction
sections that is typical of what happens when standards are created and then not
stuck with (unless there is a really good reason to change). If you read any modern
text on electromagnetic induction, you will learn that conventional current
flow is defined as going from the positive terminal to the negative terminal,
as opposed to electron flow, which is defined as going from the negative
terminal to the positive terminal. The current flow convention is justified
as representing the direction of the "flow" of holes, into which electrons move.
Circa World War II, the military used the convention of both current flow
and electron flow moving from the negative terminal to the positive terminal.
What's the big deal?," you might be asking. The big deal is the confusion caused
when applying the "hand-rules."
If you read through the Navy courses, you will see that the Left-Hand Rule is
used to determine electromagnetic induction. The Left-Hand Rule states that if you
grab a conductor with the left hand so that the thumb is pointing in the direction
of current flow (negative to positive here), the direction that the other four fingers
wrap around the conductor indicates the direction of the magnetic field. Of course,
a convention is also needed to define the direction of the magnetic field. Fortunately,
that convention has not changed over time - magnetic field lines exit the north
pole and reenter the south pole. The Left-Hand Rule is handy for predicting the
direction that a motor will rotate or a solenoid plunger will move when a current
is applied, as well as for predicting the direction of current flow (polarity) that
will result when a conductor is moved through an existing magnetic field. Get it
wrong, and everything happens just the opposite of what you expect.
I learned that the Right-Hand Rule applied for electromagnetic induction way
back to my earliest days of high school electricity classes, so it was a bit disconcerting
to see the Left-Hand Rule proclaimed throughout the Navy manuals. Indeed, it caused
me to doubt the robustness of my memory. It was one of those sickening moments when
I suddenly wondered how many times I might have confidently spouted the Right-Hand
Rule as being applicable when I should have said the Left-Hand Rule. I knew in my
head that I had to be right, but who could argue with the authority of the United
States Navy in a time of war?
An extensive search on the Internet turned up a plethora of examples of both
the Left-Hand Rule and the Right-Hand Rule being used for the same task. Yikes!
I then began digging out my college text books to find the truth, at least as I
had learned it. A great relief came over me when I discovered that in fact all of
my learning had been with using the Right-Hand Rule. Where, then, could the confusion
lie? Of course, it was with each author's definition of current flow direction.
Indeed, when the Navy's old convention is used, the Left-Hand Rules predicts the
same result as my familiar Right-Hand Rule with contemporary convention. Whew!
My trepidation was still not entirely relieved, however, because in going back
and looking at some of those aforementioned websites (including a few unnamed, but
highly regarded ones) there was no stipulation of the older negative-to-positive
current flow convention being used when the Left-Hand Rule was invoked.
One site even has a trés cool animation of a left hand moving in to grab a wire
with its thumb pointing in the indicated direction of current flow, and
then showing the resulting magnetic field - without ever mentioning that it must
be defining current flow as negative-to-positive. Scary!
In order to put my mind fully at ease, I descended the stairs to the official
RF Cafe laboratory and set up a simple experiment to prove to myself that the handy-dandy
Right-Hand Rule did in fact apply. The setup and results can be found
here.
To summarize, I used a DC current supply to feed about 150 mA into a small spool
of 30 gauge magnet wire, then held a navigational compass nearby to note the direction
of the magnetic field lines. Sure enough, everything turned out as expected.
Here are the conditions for the Right-Hand Rule:
- Current flow is from the positive terminal to the negative terminal.
- Magnetic field lines exit the north pole and reenter the south
pole.
- Wrap the right hand around the conductor with the thumb pointing
in the direction
of current flow, and the fingers will point in
the direction of the magnetic field lines.
In order to not add to the confusion of other poor souls searching for the truth
in electromagnetic hand-rules, a disclaimer is included on all the pages of the
replicated Navy manuals dealing with the Left-Hand Rule.
All those thousands of dollars of tuition and countless hours of studying have
finally paid off. Today, I are an engineer.
* CENDI is an acronym of the first letters of the participating
agencies:
Commerce Energy Environmental Protection Agency National Aeronautics and Space
Administration National Libraries of Agriculture, Education, and Medicine Defense
Interior
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