Chronometers were, and still can be, used to find position at sea. The Earth is effectively a giant clock rotating once in 24 hours, and because of this celestial objects appear to move in the sky. If you have an accurate table of positions of these objects relative to a specific place on Earth, and you know the exact time at that place, you can determine your position anywhere in the world by comparing the angle a celestial object makes with the horizon and comparing it to what that angle should be at the reference point. Greenwich England, site of the Royal Observatory of Greenwich in times past, has become that worldwide reference point. The Nautical Almanac, published yearly, is a table of positions for Greenwich. Click here for the Royal Observatory's Longitude page.

The search for an accurate means of determining time, and thus Longitude, at sea is the subject of the best selling book Longitude, by Dava Sobel. In 1715 the English Parliament established a prize of £20,000 for a method of "Discovering" the longitude at sea. Longitude recounts the story of John Harrison, an English carpenter, who was the first person to build a timepiece accurate enough at sea to win the Longitude Prize (he technically was never awarded the prize, but did receive the money from Parliament). Prior to his accomplishment, scientists, including Sir Isaac Newton, were convinced the only accurate way to measure time on a ship was an astronomical method. Harrison proved that a spring-driven timekeeper could do the job, and led the way for others to develop the standard marine chronometer. Sobel's book is fascinating, but unfortunately devoid of useful illustrations, and Harrison's timekeepers are extraordinarily beautiful. I strongly recommend The Illustrated Longitude, which is Sobel's story augmented by pictures and with commentary by William J.H. Andrewes.

Harrison's timekeepers were too complex for quantity production at a reasonable price. A great many clock and watch makers were involved in the development of a practical chronometer, chief among them being John Arnold (1736-1799) and Thomas Earnshaw (1749-1829), who were responsible for the perfection of the spring detent and temperature compensated balance. The marine chronometer pretty much reached it's final form around 1780.

A true mechanical marine chronometer has several features that distinguish it from watches and clocks. First, its escapement uses a "Spring Detent," the device which, under control of the oscillating spring balance, releases the energy of the chronometer to move the hands. The detent allows the balance to swing free of influence from the rest of the mechanism for most of its travel. Secondly, the balance spring is formed in a helical, rather than spiral, shape, and is "free sprung", that is, unlike watches, it has no mechanism to adjust the rate by changing the vibrating length of the balance spring. All corrections to rate are done by adjusting the weights on the balance, a very finicky procedure. Third, the tension of the main spring is equalized by a fusee. In common with most other spring balance timekeepers it has a balance that compensates for changes in temperature.

The most accurate mechanical clocks are weight driven because weights provide a constant force. Weights are nearly impossible to use on a rolling, pitching ship, however. Likewise, pendulums, which are kept oscillating by the constant force of gravity, are the most accurate regulators, but again are impossible to use on a moving ship. Chronometers therefore must be spring driven, and use a spring balance to keep the time. The problem with springs as power sources is that they lose tension as the run down, so a decreasing force is applied to the mechanism. To compensate, Harrison used a fusee, a cone shaped device that has a spiral groove cut on its surface. A fine chain, made like a miniature bicycle chain, connects the fusee to a barrel containing the spring. As the spring winds down, it winds the chain from the fusee onto the spring barrel, and as the chain unwinds from the fusee, it pulls on larger and larger sections of the fusee, equalizing the torque. The fusee transmits the going force to the rest of the mechanism. Although he used the fusee in his marine clocks, Harrison didn't invent it. The fusee had been known for a long time at that point.

When you wind the mechanism of a fusee-driven clock, you turn the fusee backwards, stopping the clock. This also happens with weight-driven clocks. Spring-driven clocks and watches that don't use a fusee are not affected by winding. To avoid the error this stoppage causes, Harrison invented something called maintaining power. He put a leaf spring inside the fusee, and the fusee drives the rest of the mechanism through the maintaining power spring, keeping it under tension. When the fusee is wound, a ratchet anchors the maintaining power and it delivers enough energy to run the chronometer during winding. Harrison applied this invention to weight driven clocks as well.

Temperature effects mechanical clocks, and accurate ones must compensate for this effect. Temperature causes pendulums to change their length, affecting their time. Harrison invented a gridiron pendulum that uses the different expansion rates of several metals to compensate for temperature changes. The elasticity of the balance springs used in chronometers, watches, and non-pendulum clocks varies according to temperature, causing rates to change. Harrison compensated for this with a complicated gridiron similar to that of his pendulum clocks that affected the length of the balance spring. Later makers developed a bi-metallic balance wheel composed of steel and brass. Weights on the arms of the balance move in and out with changes in temperature, changing the force required. Unfortunately, the motion of the weights doesn't exactly match the tension change in the spring, leading to what is called the "Middle Temperature Error". If a chronometer with a plain "Earnshaw" balance is adjusted to keep perfect time at 40° and 90° F, then it will gain several seconds per day at 70° F. Many ingenious systems of "Auxiliary Compensation" were developed to reduce or eliminate this error, but most introduced their own problems, chief that they usually were finicky to adjust and keep in order. The problem wasn't resolved until the invention of a special steel, Elinvar, that eliminated the change in elasticity of balance springs made from it. Another steel, Invar, was developed at the same time which basically eliminated changes in the balance wheel. Two Twentieth Century balance designs based on these inventions, the "H" shaped "Integral" balance, and Hamilton Watch Company's uncut ovalizing balance eliminated temperature variations.

A typical chronometer is mounted in a three piece box that has a lid which opens so you can see the face through a glass pane, but not touch the chronometer itself. This was designed to protect this essential timekeeper from unauthorized meddling. Until WWII the middle tier, which has the glass top, could only be opened by someone with a key, and only one or two people on board had one. The key had to be used once a day to open the box so the chronometer could be wound. This was done at the same time every day, and reported to the captain. Chronometers on warships were wound around 8:00 am, and on merchant ships before Noon. Chronometers used at sea can run 56 hours on one winding, but are wound everyday. There is an up-down indicator which shows the state of wind, and tells when the instrument needs to be wound. Most ships had two or three chronometers, whose times were compared, so that any problem with one could be detected. Redundancy wasn't invented by NASA. 

Text and some pictures copyright Norman Bliss 2002

Page created 6/20/02.

Modified 12/13/11