Why mechanical watches lose time.

Your watch is a small mechanical oscillator. It tries very hard to keep a constant frequency. Several things in the world push against it — temperature, gravity, magnetism, age, the watch's own internal physics. These are the reasons it drifts and the ways watchmakers push back.

The watch's job

A mechanical watch relies on its balance wheel oscillating at a fixed frequency — 21,600 BPH on the Seiko NH-series, meaning 6 oscillations per second. If the balance wheel oscillates faster than this, the watch gains time. If slower, it loses time. Anything that changes the balance wheel's frequency changes the watch's rate.

The balance wheel's frequency is determined by two things: its moment of inertia (essentially its mass and shape) and the stiffness of the hairspring attached to it. The wheel is precision-machined and doesn't change. The hairspring, unfortunately, changes for at least four different reasons.

Reason 1 — Temperature

The hairspring is metal. Metal expands when heated and contracts when cooled. A warmer hairspring is slightly longer, and a longer hairspring (under tension) is slightly softer. A softer hairspring makes the balance wheel oscillate slightly slower. A warm watch runs slow.

The effect is small but measurable. A 10°C temperature change can shift the rate of an unprotected watch by 5–10 seconds per day. On your wrist in summer (skin at ~32°C) versus on your bedside table at night (room at 18°C), the watch experiences this swing every day.

Modern watchmaking solves this with temperature-compensating hairspring alloys. Nivarox (used in Swiss movements), Spron 510 (Seiko's proprietary alloy used in the NH-series), and silicon (used in some premium Swiss calibres) all have low thermal expansion coefficients. They change length far less than ordinary steel does across the same temperature range. A modern NH35 might drift by 1–2 seconds per day across a normal temperature swing — almost invisibly to the wearer.

Reason 2 — Position

When a watch sits dial-up on a table, gravity pulls the balance wheel straight down through its axis. When the watch is on your wrist with the crown pointing down, gravity pulls the balance wheel sideways. The geometry is different. The balance wheel's pivot bearings are stressed asymmetrically. The watch's rate shifts by a small but real amount.

This is called positional variance, and it's a fundamental limitation of mechanical watchmaking. Different positions produce different rates. A well-regulated NH35 typically shows a 5–15 second daily-rate variance across all five test positions (dial-up, dial-down, crown-up, crown-down, crown-left, crown-right). A chronometer-grade Swiss watch achieves under 5 seconds of variance, at significantly higher manufacturing precision.

The dial-up position usually shows the highest amplitude (the balance swings widest) and the most consistent rate. The crown-down position usually shows the lowest amplitude and the most rate drift. This is why your watch may "run differently" if you wear it tighter on your wrist (more dial-up) versus looser (more crown-down).

Reason 3 — Magnetism

Metal hairsprings react to magnetic fields. A strong enough field can magnetise the hairspring, which makes adjacent coils stick together and effectively shortens the spring's working length. A shortened hairspring is stiffer, which makes the balance wheel oscillate faster, which makes the watch run fast.

A heavily magnetised watch can gain 30+ seconds per day. The visible symptom: the watch was running well, then suddenly started gaining significantly with no explanation.

The fix: a demagnetiser. A small electromagnetic device — about $20 AUD for a hobby version — passes the watch through an oscillating magnetic field that randomises the magnetisation in the hairspring back to zero. One pass through a demagnetiser solves most magnetism issues in seconds.

What magnetises a watch in daily life? Strong speakers (the back of an unshielded subwoofer), MRI machines (don't wear a watch into one), some power transformers, magnetic clasps on purses. Most modern watches are magnetically shielded enough to handle phone speakers and other normal exposures. If your watch suddenly speeds up by 20+ seconds per day, magnetism is the first thing to check.

Reason 4 — Age and lubrication

A watch's gear train and escapement run on small amounts of oil. The oil reduces friction between bearings, between the pallet stones and escape wheel teeth, and inside the mainspring barrel. Without oil, friction increases. Increased friction means less power transferred to the balance wheel. Less power means lower amplitude. Lower amplitude means greater rate variance.

Watch oil degrades over time. Modern synthetic oils last 5–10 years; older mineral oils degraded faster. As the oil thickens or dries, the watch's rate becomes less stable. Eventually parts begin to wear because of insufficient lubrication.

This is why mechanical watches need service every 5–10 years. The service replaces the oils, cleans the parts, regulates the watch, and ensures the next decade of running is as good as the last. Skip service for long enough and the watch will eventually run unreliably or stop entirely.

Reason 5 — Mainspring torque variation

The mainspring delivers more torque when fully wound than near the end of its reserve. More torque = wider balance swing = slightly different rate. Most modern movements (including the NH-series) try to engineer a near-flat torque curve, but the effect can't be completely eliminated.

You'll see this on a timegrapher: rate measured immediately after winding may differ by 2–5 seconds per day from rate measured after the watch has been running for 24 hours. The watch is running at a different point on the mainspring's curve.

For an automatic watch worn daily, this rarely matters — the rotor keeps the mainspring near fully wound, so the watch never experiences the steep end of the torque curve. For a manual-wind watch, you may notice your watch keeps better time on the first day after winding than on the second.

How regulation corrects all of this

When we regulate a watch on the timegrapher, we adjust the effective length of the hairspring to compensate for whatever's pushing the rate off. We can't change the temperature behaviour or the position dependency, but we can shift the average rate to zero across whatever conditions the watch is going to live in.

The standard regulation approach:

1. Measure rate dial-up, fully wound. Adjust regulator to bring this within ±2 s/d.
2. Measure crown-down (the position with most rate drift). Note the variance.
3. Adjust if needed to keep both positions within ±5 s/d of each other.
4. Re-measure after 24 hours running (lower mainspring torque). Confirm rate hasn't shifted dramatically.

A well-regulated NH35 emerging from this process should keep time within ±5 seconds per day in normal daily wear. That's enough that you'll need to sync the watch against your phone every week or two, but you'll never feel the watch is "wrong."

What you can do as an owner

If your mechanical watch is drifting more than you'd like, try these in order:

Demagnetise it. Magnetism is the most common cause of sudden accuracy problems. Any local watchmaker can do this for $5–$10 in a few seconds. Hobby demagnetisers are cheap and worth owning.

Wear it differently. Try sleeping with the watch in different orientations (face up on the bedside table, face down, crown up, crown down). Different positions deliver different rates. If your watch is consistently fast and you've been sleeping with it dial-down, try dial-up.

Service it if it's been five years. Old oils cause slow drifts that worsen over time. A service brings the watch back to factory-comparable performance.

Re-regulate. If a watch has run for years, the hairspring may have settled in a slightly different position. A trip to a watchmaker (or the bench) for a regulation pass will bring it back into spec.

The honest verdict

Your mechanical watch loses time because it's a mechanical oscillator made of metal, sensitive to temperature, position, magnetism, and lubrication state. The marvel isn't that it drifts. The marvel is that it drifts by only a few seconds per day across all of these effects, on a device that fits on your wrist and contains nothing electronic.

A regulated NH35 keeps time to within ±5 seconds per day. That's an error of 0.006%. The same accuracy as a kitchen timer; an order of magnitude better than the human heartbeat as a clock. For a piece of metal-on-metal mechanism the size of a coin, it's astonishing engineering.