The Marine Chronometer – A Mechanical Masterpiece

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We’ve studied the nascence of the marine chronometer. Examined the tireless work of John Harrison and the disgraceful way he was treated by the Board of Longitude. It’s time now to see the instrument in its present state.

Just a little bit of recapping. In 1765, Pierre Le Roy, who’s father, Julien, had been clockmaker to Louis XV, with Pierre following in his father’s footsteps in this regard, invented the compensation balance. Incidentally, and I apologize if I sound pernickety, but the item we’re discussing is always named simply a ‘balance.’ Never ‘balance wheel.’ However, Pierre’s invention consisted of weights placed on the balance rim and/or the crossings, or spokes. These would move around the rim and would change the moment of inertia.

Then the irascible, but brilliant Thomas Earnshaw invented his bi-metallic balance. This consisted of the steel rim, inside which was pressed a brass insert. The brass and steel expanded and contracted at different rates, much like Harrison’s gridiron pendulum. This became the standard balance and helped with temperature compensation no end.

It was found, however, that it suffered from what is known as middle temperature error. The chronometer could be regulated to keep perfect time in cold temperatures and again in a heated atmosphere, but when the temperature was in between these two points, the timing varied. In 1776, John Arnold, when he wasn’t at war with Earnshaw, invented the helical balance spring and this, together with Earnshaw’s bi-metallic balance became the standard for over a hundred years, particularly when used with Earnshaw’s spring detent escapement, or Arnold’s pivoted type. The latter saw some success on the Continent, but in England, the spring detent held sway.

Then, in about 1900, Charles Guillaume developed a nickel-steel alloy which he named Elinvar. This has a near-zero coefficient of expansion and is invaluable for spring making. Middle temperature error was almost completely done away with, and the chronometer that we know today came into being.

They were, however, hideously expensive. Being instruments that had to keep perfect time, they couldn’t be mass produced like other types of clock. In fact, it wasn’t until the great American clock company, Hamilton, in the early to mid 20th. century that mass production methods were developed.

There are basically two types of chronometer. The 2 day and the 8 day. The 2 day has to be wound every day, while the 8 day must be wound every other day.

Incidentally, don’t confuse the chronometer with the ship’s clock. They’re entirely different animals. The ship’s clock didn’t appear until late in the 19th. century, and it was fitted with a strike mechanism that struck ship’s time. One bell for 12.30, two bells for 1 o’clock, three bells for 1.30, etc., all the way round to eight bells at four o’clock, when it would start its cycle again. But it was most certainly not a chronometer.

Strictly speaking, a chronometer is what it is only if it has a spring or pivoted detent escapement. It is true that the detached lever escapements made by the Swiss are of such high quality now, that they keep extremely fine time and are occasionally called chronometers.

But, like any other clock or regulator that has the function of timekeeping alone, there are no other complications, such as strike, chime, moonwork, or anything else that would put stress on the time train in lifting levers or detents to allow for these features to operate.

All chronometers, too, are fitted with fusee mechanisms and maintaining power, both of which we’ve discussed. The movements are fully jewelled and the workmanship exquisite. The dial carries the hour and minute rings, of course, and there’s a small seconds chapter ring, usually just below the 12 o’clock position.

These days, quartz clocks have taken over. It’s true that they’re extremely accurate, but soulless. They can’t compare to the magic of the mechanical movement.

Mike Bond’s going on about his clocks again! His site’s full of them and well worth a visit:

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