Everything for Electronics
Posted in: Developing Perspectives (November 2015)

Wanted: Magnetic Free Zone

By Bryan Bergeron

For example, one of my interests is rebuilding vintage mechanical pocket watches. If you own a mechanical watch, you know that a magnetized watch will run abnormally fast. Well, I have a pocket watch on my desk that constantly gains time. I was at a loss to understand how the watch could become magnetized simply sitting on my desk. Well, using an inexpensive pocket compass, I was able to verify that the watch was being magnetized by a pair of scissors in a drawer directly under the watch. Opening and closing the drawer several times a day was enough to magnetize the watch — just as running a permanent magnet over a screwdriver can transform it into a magnet.

The discovery with my pocket watch led me to search for a magnetic free zone in my house. It was, in short, difficult. In my office, I have a dozen super magnets to hold papers on my white board. Then, there’s the unshielded speakers on the wall. In my kitchen, I was surprised to learn that some of the flatware was magnetized. On my dresser, I found my steel collar stays and magnet sets. It seemed my compass never really settled on magnetic North, given the various motors and electronic gadgets around my place.

In retaliation, I purchased a few degaussing machines from eBay, where they can be had for about $10 and up. First up was the fixed magnet combined magnetizer/demagnetizer. These devices work great as magnetizers for long thin objects such as screwdrivers, but are useless in reversing the process.

Next, I tried the generic Chinese-built “blue box” demagnetizer — essentially an AC solenoid without the moving parts and a momentary on switch. You place the screwdriver or other object you want to demagnetize on top of the box and press the button, which energizes the core with 110 VAC. Then, you slowly move the object away from the unit as far as you can before releasing the switch. The iron molecules within the tool or other object should be randomly aligned, and therefore non-magnetic. This solution was affordable, reliable, and consistent.

Given that I was looking for a solution on eBay, I also had a serendipitous find — an old US made “instantaneous demagnetizer” tool by Magna Flux ($20). This tool uses a capacitor discharge to quickly ramp down the magnetic field after it’s been built up. Like the blue boxes, it did the job. Moreover, there is no need to move the object to be demagnetized while the AC field is energized. Just press and release the button. The capacitor circuit takes care of decreasing the magnetic field.

Of course, if you decide to demagnetize your tools and mechanical watches, set up a safe area away from anything remotely resembling a magnetic data store. Don’t think of using a demagnetizer around your credit cards or your DAT collection.

With the magnetics out of the way, I’m left to puzzle over why a mechanical watch would run faster when magnetized. Is it somehow more efficient because of decreased friction? Are Eddy currents somehow imparting energy to the mainspring? Could magnetized motors somehow run more efficiently? If you have the answer, please drop me a line. NV