This highly precise wall clock was the successor of the clocks made by George Graham and John Ellicott in the 18^th century. The square head is topped by egg-and-dart moulding. The mercury pendulum can be seen in the rectangular case with its glass front. The clock ends in a triangular bracket decorated with gadroons.

The silver square clock has three dials with the numbers embossed in black. The largest dial indicates the minutes in Arabic numbers and contains the other two positioned one above the other. The upper dial contains the second hand; the lower one shows the hour in Roman numbers. The signature of the watchmaker – A Johannsen London – is in the centre.

The highly precise pendulum is mercury. It is weighted using a pulley system.

There are locks on the side walls of the case.

Asmus Johannsen was born in Denmark, but settled in London in around 1859. He specialised in precision clocks, particularly marine chronometers. He set up the A. Johannsen & Co. brand, which was a rival of the Victor Kullberg firm in London, particularly in terms of the quality of its timepieces. He won many prizes in Greenwich and Kew Trials. Johannsen made marine chronometers for India, Italy, Spain, Portugal, Austria and China.

Temperature changes caused pendulum rods to vary slightly in length, so in around 1715 George Graham began to study how to compensate metal to produce highly precise pendulums. In 1721 he manufactured the first mercurial compensation pendulum. On clocks with this system the metal bob of the pendulum was replaced by a vial containing mercury. Thus, the expansion of the bar (heat dilation) was offset by the high density of the mercury in the vial. This maintained the centre of gravity so that the measuring of time did not alter.

Amelia Aranda Huete