Magnetic Fields and Your Watch

Magnetism is the most common cause of sudden accuracy loss in mechanical watches. A watch that was keeping time within a few seconds per day and suddenly starts running several minutes fast is almost certainly magnetized. The good news: it is easy to diagnose and simple to fix.

How Magnetism Affects a Movement

The hairspring is the component most susceptible to magnetism. This thin spiral spring (typically 0.02mm to 0.05mm thick) controls the oscillation rate of the balance wheel. When magnetized, adjacent coils of the hairspring attract each other, effectively shortening its active length. A shorter hairspring oscillates faster, which makes the watch run fast.

The effect can be dramatic. A lightly magnetized watch might gain 10-30 seconds per day. A heavily magnetized watch can gain several minutes per day or even stop entirely if the coils stick together.

Other steel components in the movement (the escape wheel, pallet fork, and gear train) can also become magnetized, but the hairspring is by far the most sensitive due to its thin cross-section and proximity to itself when coiled.

Common Sources of Magnetic Fields

Modern life surrounds us with magnetic fields stronger than anything watchmakers faced a century ago.

Smartphones and tablets contain magnets in their speakers, haptic motors, and magnetic charging systems. The MagSafe system in iPhones generates fields strong enough to magnetize a watch in seconds if placed directly on the back of the phone.

Laptop speakers sit beneath the keyboard on many models. Resting your wrist on a laptop while typing places the watch directly over these magnets.

Bag and case clasps use neodymium magnets that are surprisingly powerful for their size. Placing a watch in a bag pocket near a magnetic clasp can magnetize it during transport.

Induction cooktops generate strong alternating magnetic fields. Standing near one while cooking exposes a wrist-worn watch to significant magnetism.

MRI machines produce the strongest magnetic fields encountered in daily life (1.5 to 3 Tesla). A mechanical watch must be removed before entering an MRI room. The field will not only magnetize the movement but can physically damage components.

Security gates at airports and stores produce weaker alternating fields. Brief passage through these is generally not a concern, but repeated exposure over time can have a cumulative effect.

How to Test for Magnetism

The simplest test uses a standard magnetic compass. Hold the watch within a few centimeters of the compass. If the compass needle deflects noticeably, the watch is magnetized. Rotate the watch around its axes. A magnetized watch will cause the needle to follow it.

A smartphone compass app can serve the same purpose, though dedicated compass needles are more sensitive.

For a more precise assessment, a gauss meter (available inexpensively) measures the actual field strength. Any reading above 50-60 gauss on the movement indicates magnetization that will affect timekeeping.

The most practical test is simply checking accuracy. If your watch was running within specification and suddenly gains significant time (particularly more than 15 seconds per day), magnetism is the most likely cause. Use the accuracy tracker to monitor rate changes over time.

Demagnetizing a Watch

Demagnetizing is straightforward and does not require opening the watch. A watch demagnetizer (a small electrical device that generates a diminishing alternating magnetic field) costs between $10 and $30 and takes about five seconds to use.

To demagnetize: turn on the device, slowly bring the watch toward the center, hold it there for two to three seconds, then slowly withdraw the watch at least 30 centimeters away before turning off the device. The key is the slow withdrawal. Pulling the watch away too quickly or turning off the device while the watch is nearby can re-magnetize it.

Any watchmaker can demagnetize a watch in seconds, often at no charge. It is a routine service operation.

Antimagnetic Watches

The watch industry has developed several approaches to magnetic resistance.

Soft iron inner cases (Faraday cages) shield the movement from external fields. Rolex Milgauss and IWC Ingenieur use this approach. The soft iron absorbs magnetic flux before it reaches the movement. The tradeoff: no exhibition caseback is possible.

Silicon hairsprings are inherently non-magnetic because silicon is not a ferromagnetic material. Brands using silicon hairsprings include Rolex (Parachrom, while not pure silicon, incorporates paramagnetic alloys), Omega (Si14), Patek Philippe (Spiromax), and Swatch Group's brands using Nivachron.

Omega's Master Chronometer certification requires the movement to maintain accuracy while exposed to 15,000 gauss. This is achieved through silicon components and non-ferromagnetic alloys throughout the movement. For comparison, ISO 764 (the standard for antimagnetic watches) only requires resistance to 4,800 A/m (approximately 60 gauss).

Prevention

The simplest prevention is awareness. Avoid placing your watch on top of phones, laptops, or near magnetic clasps. When storing a watch, keep it away from electronics.

If you work in an environment with strong magnetic fields (electrical engineering, MRI facilities, industrial motors), consider a watch with demonstrated magnetic resistance or wear a quartz watch, which is unaffected by magnetism.

For collectors with multiple mechanical watches, a periodic compass test takes five seconds and catches magnetization before it becomes a problem.