Marine Chronometer

Overview

A marine chronometer is a spring-driven clock built to keep mean time accurately enough, over weeks at sea, to find a ship's longitude. Before satellite navigation, a navigator compared local time (taken from the sun) against the chronometer's reading of a reference meridian; every four minutes of difference is one degree of longitude. That demanded a timekeeper that held its rate through the rolling, damp, temperature-swinging environment of a ship, the problem John Harrison and his successors spent the 18th century solving.

The instrument is organised for stability. The Movement carries the timekeeping train and the detent escapement, and it sits in a turned Gimbal Mount bowl that stays level as the vessel rolls. A Power-Reserve Indicator dial shows how much running remains, the time is read on the Dial & Hands, and the whole instrument lives in a cushioned Storage Box and is wound with its dedicated Winding Key.

How it works

Power is stored in a coiled mainspring, but a mainspring delivers more torque when fully wound than when nearly run down. The fusee corrects this: a chain wraps from the barrel onto a spiral-grooved cone, and as the spring weakens the chain pulls from the cone's larger radius, so the torque reaching the train stays nearly constant. A maintaining-power detent keeps the train driven during the few seconds the fusee is being wound, so the clock never stops.

The going train ends at the detent escapement. Unlike a lever, the spring detent impulses the balance only once per cycle and otherwise leaves it detached and swinging almost freely, which is why detent chronometers are so stable. The balance itself is a bimetallic split rim that changes diameter with temperature to cancel the way heat softens its helical balance spring; small screw weights on the rim let a rate adjuster poise and time it. The gimbal keeps the escapement working in its preferred horizontal plane regardless of the ship's motion.