Fourth wheel

The fourth wheel is part of the wheel train of a watch movement. It is labeled “Z5” in the illustration at right.

It is named for its position (fourth) in the wheel sequence of the going train. It is also sometimes called the seconds wheel, though this is easily confused with another gear, the second wheel.

The fourth wheel moves the seconds hand in watches with a subsidiary (“small”) seconds dial and has the corresponding gear ratio.

• Wheel train