Stan Trout, President, Spontaneous Materials
Not long after I completed my series on the seventeen mistakes people make with permanent magnets, I received a suggestion for an eighteenth. This mistake involves confusing the polarity of magnets, a topic I mention early in my bootcamps. But I never made the leap to connect this topic to my list of mistakes. So, I appreciate the recommendation. Here is my description of the 18th mistake.
Two things we know about magnets are: they have both north and south poles, and that opposite poles attract. A compass is a nice example of this concept, since the needle is free to pivot and turn in reaction to a magnetic field. The arrow of the compass we are told, points north. It does so in response to the magnetic field originating from the magnetic pole located near the Earth’s geographic north pole, at least when in the northern hemisphere. Since the Earth’s magnetic pole attracts the arrow of the compass, either the Earth’s pole or the arrow of the compass is a north pole and the other is a south pole. The question is which is which? It turns out that there are three conflicting, yet equally valid ways of looking at this situation.
One approach is to say that the tip of the compass arrow is a north pole, since it points toward the geographic north. That means the magnetic pole nearest the Earth’s geographic north pole must be a south pole, since opposites attract.
The second approach is to say that the magnetic pole nearest the Earth’s north pole is a north pole. That means the tip of the arrow must be a south pole.
Which is correct? It turns out that the answer is not as important as you might imagine. The critical concern is consistency. In other words, first define the terms north pole and south pole, and then rigidly hold to that definition.
My approach is to use the first definition. In this case, we are really saying the north pole of a magnet is a north seeking pole, meaning that it is the pole which turns toward the geographic north pole when the magnet is allowed to rotate freely. In my seminars I show a picture of the Earth with a bar magnet superimposed on it, which is shown below. At first look the bar magnet might appear upside down, since its south pole is up, and the north pole is down, in contradiction to the geographic poles. But it must be this way to be consistent with our definition of north and south poles.
What happens if we use the other definition? The magnetic pole nearest the Earth’s geographic pole is now a north pole, the bar magnet in my figure is flipped and the tip of the arrow in the compass is now a south pole. This approach works, too; the only difference is that all polarities are reversed compared to the first definition.
In practice, we occasionally interact with people who use the second definition, and who may not be sensitive to our two different definitions of polarity. (One might say we are polar opposites.) It can cause some confusion at first, until we recognize the source of the mix-up. Once it is clear how the poles are defined, there should be no problems, again as long as we are consistent.
My undergraduate textbook on electricity and magnetism offers a third way to handle the conundrum. Authors Corson and Lorrain claim the whole concept of magnetic poles is completely fictitious. The only thing that really matters is magnetic flux. As someone who learned about magnetic poles in the first grade, I puzzled over this comment a long time before I could accept it. However, my acceptance was temporary, just for the duration of the course. Perhaps this is the most elegant way to handle the question of polarity, to say that the whole concept is unnecessary. However, this approach could prove difficult to explain to customers, who likely believe in poles!
So, we now have eighteen documented mistakes. Does anyone have number 19?
Dr. Stan Trout has more than 35 years’ experience in the permanent magnet and rare earth industries. Dr. Trout has a B.S. in Physics from Lafayette College and a Ph.D. in Metallurgy and Materials Science from the University of Pennsylvania. Stan is a contributing columnist for Magnetics Business & Technology magazine. Spontaneous Materials, his consultancy, provides practical solutions in magnetic materials, the rare earths, technical training and technical writing. He can be reached at firstname.lastname@example.org.