Now the reason I’m (painfully) sticking a wrench to my hand with a 4K gauss magnet is because it’s easier to understand how powerful the magnet is from a picture, than from expressing field strength in things like gauss or tesla, which are more abstract. There are actually two kinds of magnetic fields: B, and H. The so-called B field is a measure of field strength in free space, and is measured in tesla, or gauss. One tesla = 10,000 gauss; a typical refrigerator magnet is about 50 gauss, and our test magnet, at about 4,500 gauss, is nearly half a tesla, which is more than enough to fry any conventional watch. Obviously, the plain steel balance springs used in watches before the advent of Nivarox type alloys would be incredibly vulnerable to external magnetic fields, but even a watch with a standard modern Nivarox balance spring would be instantly rendered unusable by a magnet as powerful as the one we used in our test. Even with the high antimagnetic ratings of both the Milgauss and the Omega >15,000 Gauss, the idea of applying such a powerful magnet was a little alarming. It’s one thing to know your car has an air bag; it’s another thing to deliberately run it into a brick wall to find out if it works like it’s supposed to.

By the way, you may have heard resistance to magnetism in watches expressed in amperes per meter, sometimes shortened to A/m. This unit is used to express the strength of the other magnetic field, the so-called H field – as is its equivalent, the oersted. The H field is basically the strength of the B field, but including its effects inside a material on the overall field. Fortunately for those of us comparison shopping for antimagnetic watches, in air or a vacuum the B and H fields, and therefore, the gauss and the oersted, are about equal. The conversion from oersted to A/m is a little more involved but to give a concrete example, the famous IWC antimagnetic Ingenieur 500,000 A/m could resist a field of almost 7,000 oersted/gauss. A rating of 1,000 gauss resistance is equal to about 80,000 A/m.

As you can see, the B and H fields aren’t really different so much as they are the same phenomenon seen from different perspectives.

Fun fact: your brain produces a field of about one picotesla, or  0.000000000001 tesla, which is so weak you need a neat gadget called a SQUID (Subatomic Quantum Interference Detector) to pick it up.