Mechanical Clock

A mechanical clock lets a falling weight turn a train of gears, and an escapement lets the train move forward only one small step at a time. The first ones were built in Europe at the end of the thirteenth century, large iron machines in church towers that struck the hours on a bell.

What it is

A mechanical clock divides time into equal beats and counts them with gears. The earliest kind, made from the end of the thirteenth century, was a large frame of wrought iron in a church or town tower. It was driven by a heavy stone weight and controlled by a verge and foliot, a rocking bar that let the wheels turn only in small, regular steps. Many had no dial at all: they struck a bell at the hours, and people learned the time by ear.

At Salisbury Cathedral a clock of this type still runs in the nave. It is usually identified with a clock mentioned there in 1386, and on that basis it is often called the oldest working clock in the world; the dating rests on indirect evidence, and other clocks are put forward for the title. Its wheels sit in a four-post iron frame held together with wedged tenons instead of bolts. One weight drives the going train, which keeps time; another drives the striking train, which rings the bell.

How it works

The weight hangs on a rope wound round a wooden barrel. As it slowly falls, it turns the barrel and the great wheel fixed to the same arbor. The great wheel drives a small pinion on the next arbor, which carries another wheel, and so on up to the escape wheel. Each step from wheel to pinion makes the next arbor turn faster.

The verge escapement seen from the side, where the teeth round the crown wheel’s rim line up in one column: the wheel pushes the two pallets in turn, and the weighted foliot swings back and forth on top of the verge.

Left to itself the weight would spin the train and fall in seconds. The escapement stops that. The escape wheel is a crown wheel, with saw-shaped teeth standing out from one face. In front of it hangs the verge, an upright rod with two small flags, the pallets, one meeting the teeth at the top of the wheel and one at the bottom, set at roughly a right angle to each other. A tooth pushes the upper pallet aside and turns the verge; that brings the lower pallet into the path of a tooth on the far side of the wheel, which pushes it back the other way. The verge therefore rocks to and fro, and the wheel advances by half a tooth at each swing one way, a whole tooth for each swing there and back.

The foliot is a horizontal bar fixed across the top of the verge, with a small weight hung near each end. Its inertia slows each swing. Moving the weights outward makes the beat slower and the clock lose; moving them in makes it gain. The foliot has no natural period of its own, so the rate also changes with the force of the driving weight, with friction and with the state of the oil, and a typical error was around a quarter of an hour a day. The keeper reset the clock against a sundial and wound up the weights every day.

How to build it

A working model of a verge-and-foliot escapement in plywood and hardwood dowel can be made with hand tools. It ticks and shows how the escapement controls a falling weight; with no dial it does not tell the time. What follows is an overview of the stages: for tooth shapes and arbor spacings, work from a published, dimensioned plan for a wooden escapement model.

A lantern pinion: round rods between two discs, easy to make by hand, mesh with the teeth of the larger wheel.

Build the frame. Two uprights about 40 cm tall joined by cross-bars, with holes for three horizontal arbors one above the other at the spacings given by the plan. Use dowels as arbors and line the holes with short pieces of brass tube so they turn freely. Screw the frame to a heavy base or a wall bracket so it cannot tip.

Make the train. On the bottom arbor go a drum for the cord and the great wheel; on the middle arbor a lantern pinion of dowel rods and a second wheel; on the top arbor another lantern pinion and the crown wheel. Each wheel must mesh with its pinion without binding or slipping, so cut the teeth to the plan’s profile and file them smooth.

Make the escapement. The crown wheel needs an odd number of teeth, 15 is common, standing out from one face: cut them on the edge of a short ring of thin plywood or hardwood and glue the ring to the face of a disc. Mount the verge, a straight dowel, vertically in front of the wheel, with two thin pallets about 90 degrees apart at the heights of the top and bottom of the wheel. The odd number of teeth means one pallet meets a tooth tip while the other faces a gap. Fit the foliot, a light bar about 30 cm long with two sliding weights, across the top of the verge.

Adjust without the weight. Turn the train by hand with a finger on the drum and check that the pallets alternate: a tooth at the top escapes, then a tooth at the bottom catches the other pallet, with no tooth skipped and nothing jamming. Bend or file the pallets only while the weight is off. Then wind a cord rated well above the load round the drum, tie its end to the drum and hang the weight your plan specifies, often 1 to 2 kg for a small wooden model. To set the beat, lift the weight off or support it, slide the foliot weights a little, let it hang again and count the ticks over a minute; repeat until the beat is steady.

Safety. If a pallet slips, the train runs away and the weight drops: always lift the weight off or support it before touching the escapement, and let the weight run inside a closed guide, such as a tube or a box, that stops at the floor. Keep hands, feet and pets out of its path. Clamp the work and keep hands behind the blade when cutting teeth, and wear eye protection when drilling.

History and evolution

The striking train pulls the wire, the hammer swings against the bell, and the hours are heard across the town.

Water clocks and sundials had measured time for thousands of years before mechanical clocks. The decisive step, a mechanism that let a weight fall in regular jerks, was taken somewhere in Europe in the late thirteenth century, and large public clocks are recorded in Italian cities in the first half of the fourteenth. Clocks appear in the records of cathedrals and abbeys from that time, but the same Latin word, horologium, served for water clocks too, so the first mechanical clock cannot be named. By the end of the fourteenth century large iron clocks were at work in many cathedrals; the movement of the Wells Cathedral clock, of 1392, is now in the Science Museum in London, while its dial is still in the cathedral.

The great change came in 1656 to 1657, when Christiaan Huygens, following an idea of Galileo’s, used a pendulum to regulate a clock. A pendulum swings in almost the same time whatever the drive. Huygens’s first pendulum clock erred by less than a minute a day, and his later ones by less than ten seconds. Many tower clocks were converted; Salisbury’s and Wells’s both lost their foliots for a pendulum, and Salisbury’s was given a reproduction verge and foliot when it was restored in 1956. Around 1675 a spiral balance spring gave watches a regulator with a period of its own; it brought their error down to some ten minutes a day, far behind a pendulum clock but much better than before.