Copley Clock

John Harrison’s Third Marine Chronometer

Harrison devoted nineteen years to the development of H3 which he called his 'curious third machine’ and hoped it would be accurate to about 3 or 4 seconds a week. It has 753 parts and weighs 60 pounds. Started in 1740, H3 took Harrison nearly nineteen years to build and adjust. He found that he just could not persuade the two large, heavy, circular balances to keep time well enough. Nevertheless, H3 incorporates two extremely important inventions both relevant today: the bimetallic strip (still in use worldwide in thermostats of all kinds) and the caged roller bearing, a device found in almost every modern machine

H3 has two large interconnected circular balances which are carried on roller bearings, linked by metal ribbons and controlled by a single spiral spring. The clock is powered by a spring and fusee, which is a device to keep the torque of the spring constant. It also has a remontoire (literally a rewind) which serves to also keep the torque on the escape wheel constant. The escapement is of the frictionless grasshopper variety. This clock can be seen in full working order in the Navigation room of the National Maritime Museum at Greenwich along with its brothers H1 and H2.

The Technology of H3

Harrison left no drawings of his clocks but detailed drawings were made by both E .J. Dent and Rupert Gould.

H3 is a large clock weighing 27 kg and stands 59 cm high. It contains 753 separate parts. It is characterised by twin interlinked balance wheels with a large coiled spring attached to the upper wheel and it beat seconds. Both are driven by a grasshopper escapement mounted between their shafts. Large roller bearings support the shafts and small caged rollers, judged to be the forerunners of modern roller bearings are to be found elsewhere in the train. A remontoire reset every thirty seconds is fitted and this is driven by a large spring barrel and fusee. There is a locking mechanism between fusee and remontoire, which prevents the latter from unwinding. A maintaining power is fitted to the winding spring. The clock is fitted with a temperature compensating bimetallic strip or curb, which slides on the balance spring.

The Balance Wheels

The large interlinked balance wheels are joined by adjustable crossed wires, ensuring that both balances work together without any play and also allow complete freedom of movement. The range of excursion of the wheels is about 20 degrees and the period of oscillation is one second. It is a heavy oscillator with a limited range of motion. The balance spring is a large very short very heavy spring of 1½ turns applied to the arbor of the upper balance. This oscillator was of course designed to replace the pendulum and to be impervious to a certain degree of clock movement while mounted in gimbels aboard ship.

Each balance shaft is supported on two sets of supports, one in front and the other behind. These supports are in fact long arms whose bearings are in the clock plates, but the section which supports the balance shafts are small parts of a large circle and the effect is that of a roller bearing. The arms are cross-linked by springs to balance their weight.

The Grasshopper Escapement

There are two grasshopper palates, one above the escape wheel and the other below. Each is mounted on a carrier attached to a balance shaft. They are each weighted in such a way that their pallets move away from the escape wheel teeth, and are returned to a tooth by a controlling spring. This spring also undergoes slight bending after the pallet connects with a tooth and while the escape wheel gives impulse. Once this is completed, slight counter-rotation of the escape wheel frees the pallet, which recoils because of the way it is weighted, and its movement is arrested by the spring, which quickly damps any oscillation before returning it to the next tooth, which it does accurately.

The Escape Wheel

There are 120 teeth in the escape wheel and they are of narrow pointed profile and angled in the direction of rotation.

H3 was never taken to sea. It did make a short voyage from London to Plymouth where it was kept on board ship for months. During the making of H3, Harrison was directing work on a watch, which became H4. Harrison asked the Board of Longitude to consider the watch 'as being of great service with respect to longitude' and it was this timepiece that eventually won the longitude prize. H3 was accurate to within ¾ second per day.

The Meccano Copley Clock

The Meccano version of H3 has been named in honour of the Copley Prize awarded to John Harrison by the Longitude Board for developing his navigation clocks. It cannot emulate all the features of the prototype. Nevertheless this lovely clock closely follows the appearance and function of the original as far as the style and escapement are concerned. The movement is restful and fascinating and is reliable and durable.