For The Last Time, An Antimagnetic Watch Does Not Use A Faraday Cage

And "begs the question" isn't synonymous with "raises the question" either.

One of the things I have always been stubborn about (the list is long) is terminology, in horology and in other places too. I mention people saying “begs the question” when they mean, “raises the question” because it is one of my most pet of all peeves. Question begging is a form of informal logical error known as petitio principii, in which a question assumes the truth of a premise which has not actually been shown to be true. A classic example is “Coca Cola is the most popular soft drink in the world. Therefore, no other soft drink is as popular as Coca Cola,” and question begging is therefore a form of circular reasoning. Yes, I know that using it to mean “raises the question” is common and even increasingly enshrined in dictionary definitions, which should know better, but to use “begs the question” as a substitute for “raises the question” obscures the essential difference between the two and clouds the incisive critical purpose of the former, and I hate it; a curse on the usage and all who sail in her; may the confusion be hurled into an outer darkness, where there is a raging and a gnashing of teeth.

Anyhow, I have lots of pet peeves in watch terminology as well, one of which is the nearly universal tendency to say that an antimagnetic watch – specifically one with a soft iron dial and inner case – uses a Faraday cage. It does no such thing.

A Faraday cage usually takes the form of a mesh or solid enclosure made of some highly electrically conductive material – copper, or aluminum, or some alloy of comparable or better conductivity. They were invented by Michael Faraday, in 1836, although the fact that an external electrical charge would not affect something inside a conductive enclosure was observed earlier – Ben Franklin was one researcher who noted the effect. He lowered an uncharged cork ball into an electrically charged metal can, and wrote, “ … the cork was not attracted to the inside of the can as it would have been to the outside, and though it touched the bottom, yet when drawn out it was not found to be electrified (charged) by that touch, as it would have been by touching the outside. The fact is singular.”

A Faraday cage works thanks to the fact that an electromagnetic field passing through the cage or enclosure, will induce an electrical current, which is equal in strength but opposite in direction to the incoming field. The two fields cancel each other out and anything in the interior of the enclosure is unaffected. Faraday cages can cancel out radio frequency and other electromagnetic waves, including microwaves and they have dozens of uses – “booster bags” lined with magnetic foil can be used by shoplifters when stealing RFID tagged goods, and Faraday cages are also used to prevent remote electronic snooping (they are a part of the US and NATO Tempest standards, which seek to provide better security against listening in on stray RF and other communications emissions).

What a Faraday cage does not do, is shield against static (as in a permanent magnet) or low frequency magnetic fields; a compass will still work inside a Faraday cage. For shielding against magnetic fields, you need a Mu metal enclosure.

Mu metal is an iron and nickel alloy which is named for the Greek letter mu, μ, which in electromagnetic physics represents the magnetic permeability of a material – that is, the degree to which a material provides a preferred path for magnetic field lines. The alloy has a number of proprietary formulations – for instance, as produced by the good folks at the Mu Metal Shield Corporation. The high magnetic permeability of the alloy means that magnetic field lines will pass through the Mu metal enclosure (in the case of a watch, the Mu metal/soft iron dial and inner case) rather than flowing through the movement and magnetizing the movement. There is some overlap between Mu metal enclosures and Faraday cages inasmuch as Mu metal is also electrically conductive, but Mu metal enclosures are specifically designed to shield against magnetism, and Faraday cages are designed primarily to block radio frequency interference.

To quote the Mu Metal Shield Corporation’s FAQ:

“Does MuMetal block magnetic fields?
MuMetal re-directs magnetic flux so it lessens the magnetic field’s influence on the item being shielded.  Shielding does not eliminate or destroy magnetic fields, nothing does.  It does, however, provide an easy path for the magnetic field to complete its path.  You may think of it as a magnetic field conductor or shunt. 

”Does MuMetal provide shielding at radio frequencies?
Mumetals provide interference control of H-fields [magnetic fields], DC or AC up to 100 kilohertz, and differ fundamentally from RF shielding [emphasis mine]. The permeability of our shielding alloys begins to reduce drastically near a frequency of 1 kHz.  [The radio frequency spectrum is roughly between 3 kilohertz, to up to 300 gigahertz.] MuMETAL is conductive, so it can potentially shield higher frequency (RF) fields 1-100 kHz, even though they are formulated specifically for low frequency magnetic fields. When both high frequency and low frequency fields are present, our alloys may be most effective if grounded and designed with proper RF shielding practices. Shielding for E-fields and radio frequencies (RF) above 100 kilohertz involves use of high conductivity materials such as Copper, Aluminum or certain conductive coatings.” 

I tried to explain this to a colleague of mine – young, ambitious, and full of go, as the Brits like to say and he rebuked me by saying that there were plenty of watchmakers who say that their antimagnetic watches use Faraday cages. My reply to this is that a thousand wrongs don’t make a right, that watchmakers moreover have historically been driven by empirical results rather than theory (which is to say I don’t think any watchmaker gives two bits for whether or not a Mu metal enclosure is a Faraday cage; they say it because everyone else does and it seems to make sense to consumers). I would also say that in many cases, terminology from the brands is driven by profit motive rather than technical accuracy. One of my very favorite people in the industry and a stalwart individual of great moral probity, insists on describing his brand’s remontoires as “constant force escapments” even though this can be shot down easily by pointing out that the watches, in fact, have standard lever escapements with a constant force regulating mechanism upstream and that they are no more constant force escapements than a watch with a fusée and chain. But that is a rant for another day.

Anyway I have no skin in the game nor dog in the fight, I merely mention the distinction for selfish reasons, which is that every time I see someone say “Faraday cage” when they mean “Mu metal enclosure” my gorge rises. The irritation is even more ridiculous inasmuch as advanced materials science has rendered Mu metal enclosures obsolete – they add weight and thickness, although they do have a kind of old school charm. I miss them, especially in the Milgauss. Still, if being a watch enthusiast entitles you to anything, it is chronic irritation over things that are technically irrelevant – I mean, that’s kind of my jam.