From motion to minutes
How does an automatic watch work?
An automatic watch turns the movement of your wrist into stored energy. A weighted rotor winds a mainspring; that spring powers the gears and hands, while a balance and escapement regulate their progress. The watch keeps running after you take it off, using the energy already stored inside. [3]
I’m using the NH35A here to make those connections visible. Follow the six steps below, or jump straight to the questions that brought you here.
The crown and rotor wind the spring
The crown is the small knob on the side of the case. In its winding position, turning it transmits motion through the winding mechanism to the mainspring’s central shaft, called the arbor. The rotor provides another input: its off-center weight swings as the watch moves.
In the NH35A, the automatic train uses a pawl lever and reduction wheels to turn rotor movement into winding. Both inputs replenish the same spring. [1] [5]
The mainspring stores the energy
The mainspring is a long, coiled strip of metal inside a toothed drum called the barrel. Winding bends the spring, storing energy. As it relaxes, it turns the barrel and supplies the force that runs the movement. [4]
Power reserve describes how long a watch can run without more winding. For the NH35A, TMI specifies more than 41 hours when fully wound. The percentage in my interactive is a teaching aid, not a measurement of a real watch. [1]
The gear train carries the power
The barrel drives a connected series of toothed wheels and smaller gears called pinions. Their different tooth counts create different rotation speeds. This going train transfers energy toward the escapement and provides the rotations used to display time. The gears transmit the motion; the regulating system determines its pace. [3]
The balance and escapement set the rhythm
The balance wheel swings back and forth, with a fine hairspring providing the restoring force. The escapement’s escape wheel and pallet fork work with those swings to release the train in small steps. In return, the escapement gives the balance tiny impulses that keep it moving. [4]
The NH35A runs at 21,600 beats per hour: six beats each second, or three complete back-and-forth oscillations. That is why its seconds hand appears to sweep, even though it advances in small steps. [1]
The hands turn at three different speeds
In this movement, the fourth wheel drives the seconds hand around once a minute. A set of gears beneath the dial, called the motion works, drives the minute and hour hands. The minute hand takes one hour to circle the dial; the hour hand takes twelve. They share the movement’s power and rhythm. [1] [6]
The calendar advances the date
A calendar drive linked to the hour wheel advances a numbered ring once every 24 hours. The date window reveals one part of that ring. The NH35A’s calendar cycles through 1 to 31, so shorter months need a manual correction. Its calendar parts appear separately from the going train in TMI’s parts guide. [1]
See it happen: wind the model and choose See midnight. I’ve simplified the changeover timing to make the ring’s movement easy to follow.
The everyday questions
Automatic watches, explained.
What is the difference between an automatic and a mechanical watch?
An automatic watch is a type of mechanical watch. A hand-wound movement relies on you turning the crown; an automatic adds a rotor that winds the spring as you move. [3]
Does an automatic watch need a battery?
A conventional automatic mechanical watch, such as one powered by the NH35A, needs no battery. It stores energy in a mainspring. Quartz watches use electrical energy and a quartz oscillator to keep time. [3]
Why does my automatic watch stop when I take it off?
It keeps running on its stored energy until the reserve runs out. How much energy remains depends on how well it was wound beforehand; a short or inactive period on your wrist may not replenish what it uses. [7]
How long will an NH35A run without being worn?
TMI specifies more than 41 hours from a fully wound mainspring. That is a full-wind figure, not a promise after any period of wear. Other automatic movements have different power reserves. [1]
Can I wind an automatic watch by hand?
The NH35A supports hand winding: turn the crown clockwise in its normal winding position. Other movements may differ. Follow your watch’s instructions, including unlocking a screw-down crown if fitted. [2]
Can you overwind an automatic watch?
Modern automatic movements generally use a slipping mainspring attachment. At full wind, it allows the spring’s outer attachment to slide inside the barrel instead of building more tension. There is no need to keep winding once full, and you should never force a resistant crown. [7]
Does winding the crown make the hands move faster?
Winding replenishes the energy supply. The balance and escapement regulate how quickly the train advances, so winding is not a speed control. A real mechanical watch’s rate can still vary with its winding state, position, temperature, and other conditions. [4]
Can I adjust the date at any time?
Check the movement’s instructions. For the NH35A, TMI says not to use the date-setting function between 9 p.m. and 4 a.m., when the calendar mechanism may be engaged. This refers to the time indicated by the watch, including AM/PM, not the time on your phone. [2]
Sources & illustration notes
I’ve linked the manufacturer’s specifications and operating instructions, alongside explanations of the underlying mechanisms. NH35A-specific figures come from TMI.
- TMI: NH3 Series technical guideNH35A specifications and parts layout (PDF).
- TMI: NH35 operating manualCrown winding, time setting, and date-setting restrictions (PDF).
- Federation of the Swiss Watch Industry: mechanical and quartz watchesThe energy source, regulation, and display.
- Seiko: mechanical watches, automatic and windingThe barrel, balance, hairspring, and escapement.
- Seiko: automatic winding and the Magic LeverHow rotor motion in either direction can wind a mainspring.
- Horopedia: motion workThe gearing that drives the minute and hour hands.
- Seiko: 6S28 / 6S37 instructionsGeneral automatic-winding and slipping-mainspring explanation, pp. 19–20 (PDF); not NH35A specifications.
What the interactive simplifies
This educational cutaway combines front and back layers, enlarges some parts, and uses illustrative gear shapes and tooth counts. Dotted highlights indicate functional connections.
Winding adds energy much faster than in a real watch. The model uses a 41-hour reserve scale, and its date change takes an illustrative ten minutes of watch time ending at midnight. Neither the winding input nor the calendar timing is a measured NH35A behavior.
In time lapse, the fast train and escapement stay still and the seconds hand is hidden to prevent flicker. The minute hand, hour hand, and date continue on the same accelerated clock.