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