Archives: Complications

Before artificial lighting the phase of the Moon decided whether a road could be travelled at night, when a harbour could be entered, and how long field work could go on after dusk. Tides follow the lunar cycle closely enough that a coastal sailor who knew the Moon’s age could estimate high water without tables. An almanac or the sky itself could answer the question, but neither helps under cloud or by day. Putting the count into a clock meant it was always there.

The conventional display puts two identical moons diametrically opposite on a disc, so one revolution covers two lunar months and each moon crosses the aperture in turn. Behind it sits a wheel of fifty-nine teeth, advanced one tooth every twenty-four hours by a finger driven from the hour wheel, with a sprung jumper to hold each position. The aperture is cut with two convex flanks, so that as the disc turns the painted moon is progressively masked and revealed.

That arithmetic is the limit. The mean interval from one new moon to the next is 29 days, 12 hours, 44 minutes and about 3 seconds; fifty-nine teeth for two months models it as exactly 29.5. The display therefore gains about 44 minutes a month and a full day after roughly thirty-three months, after which it needs correcting. Trains built around a 135-tooth wheel reduce the error to a day in something over a century.

A simple date mechanism advances by one every twenty-four hours and knows nothing of the calendar, so it has to be corrected by hand at the end of every month shorter than thirty-one days: five times a year, and by three days after a common February. For anyone who dated documents for a living the annoyance was constant. Thomas Mudge built a perpetual calendar into a pocket watch in London in 1762.

The memory of the mechanism lives in one component. The programme wheel, usually called the forty-eight-month cam, turns once every four years and carries forty-eight steps around its edge, one for each month of the leap cycle, each cut to a depth matching that month’s length. Once a day a large pivoted lever is released and falls; one arm feels the step presented to it, and the distance it travels decides how far it drives the date ring. Other arms of the same lever index the day star, the month star and the leap-year indication, which is simply a reading taken off the cam itself.

It is fragile by nature. A great deal of stored energy is released in an instant around midnight, and the calendar train is under load for two hours or more either side of the change, during which a correction forced through the pushers can break a lever.

The mechanism fails in 2100 because a four-year cam can only express the Julian rule that every fourth year is a leap year. It has no way of knowing the Gregorian exception that centurial years are common years unless divisible by four hundred. Every perpetual calendar built on a plain forty-eight-month cam will need to be advanced by a day on 1 March 2100, and again in 2200 and 2300, but not in 2400.

A single chronograph hand can time one event. Two horses crossing a line seconds apart, or two laps of a track, means either two instruments and two observers, or accepting that the first reading is lost when the hand is reset for the second. Joseph Thaddaeus Winnerl arrived at the answer in Paris by about 1838: two seconds hands on one axis, started together, one of which can be stopped, read, and returned instantly to the other.

The two hands sit on concentric arbors, the lower one the ordinary chronograph wheel, the upper the split wheel with its own heart-shaped cam. A lever ending in a small ruby roller rests in the notch of that cam, and it is the roller, not any gear tooth, that drags the split wheel round in step with the wheel beneath. Pressing the split pusher closes a pair of sprung pincers on the rim and stops it; the wheel below carries on, pushing the roller out of the notch and up the flank of the cam. Releasing the pusher lifts the pincers, the roller slides back down into the notch, and in doing so swings the stopped hand forward until it lies exactly beneath the running one.

The cost is friction. While the split hand is held, the train has to drag the roller round against the cam, which draws amplitude from the balance; hold it too long and the rate suffers. Better constructions add an isolator that lifts the roller clear the moment the pincers close, at the price of more parts and a more delicate adjustment.

Beginning a new timing on an ordinary chronograph takes three actions: stop, reset, start. For a navigator flying by dead reckoning, timing each leg of a course from the moment of the turn, those three presses cost seconds and attention at exactly the wrong moment. An error of one minute in elapsed time is worth about fifteen nautical miles of position.

In an ordinary chronograph the reset is locked out unless the mechanism is stopped, to protect the gearing. A flyback removes that interlock. One press lifts the brake, drops the hammer onto the heart cams of the seconds and minute counters, drives them to zero and lifts the hammer clear, without ever disengaging the clutch. Release the pusher and the count resumes from zero.

The cost is mechanical violence. The reset happens against a train still turning under power, so the cam, the hammer face and the pivots take a sharp blow every time. Tolerances must be tighter than in a standard chronograph and the adjustment is unforgiving: a fraction late and the hammer brakes the train, a fraction early and the hand does not settle on zero.

Longines filed the defining patent in June 1935 and built the function into calibre 13ZN. Surviving examples suggest the firm was already making flyback chronographs in the late 1920s, so the patent probably formalised an existing practice.

Timing a short interval with an ordinary watch means noting the seconds hand at the start, noting it again at the end, and subtracting. Astronomers, artillery officers and racing officials could not do better by stopping their watches, because a stopped watch loses the time of day. What they needed was a second hand they could start and stop at will, while the watch itself kept running.

That is what the dial above shows. The long hand from the centre belongs to the chronograph and does nothing until the top pusher is pressed; the watch’s own seconds live in the small dial at nine. The other two counters accumulate the elapsed minutes and hours, because a hand that has gone round three times looks exactly like one that has gone round once.

Underneath, a chronograph is a second gear train that can be coupled to the movement and uncoupled again. Three things have to happen in order, and one part sequences them. The column wheel is a small castellated wheel, like the top of a chess rook, advanced one step at every press. Levers for the clutch, the brake and the reset hammer rest either on top of a column or drop into the gap between two, so a single turning part decides what happens next. The reset depends on the heart cam, patented by Adolphe Nicole in 1844: a flat hammer falling against a heart-shaped cam can only come to rest in one angular position, and that position is zero.

It is not free. The chronograph draws power from the same mainspring as the escapement, so the rate shifts while it runs, and the mechanism roughly doubles the part count, adds height, and puts two more openings in the case.