This magnificent compass in a brass case was almost certainly a ship’s instrument, as it’s mounted on gimbals – pivoted supports which would keep the compass itself stably horizontal on even the wildest of seas.
It would probably have been mounted in a binnacle – a waist-high pillar on the deck of a ship which would contain various essential instruments, including one or more compasses, timers, and lamps.
It was made by A Johannsen & Co, of 149 Minories, London, who proclaim themselves on the dial as ‘opticians’ and ‘adjusters of iron ships’ compasses’. That last is particularly pertinent as iron can have a dramatic effect on the accuracy of a magnetic compass.
Early binnacles often used iron nails, which could cause magnetic deviations in compass readings. As both the development of the compass and understanding of magnetism progressed, ship-builders began to pay greater attention to binnacle construction to avoid compass disturbances caused by iron.
With the increased use of iron-clad ships in the mid 19th century, the magnetic deviation observed in compasses became more severe, and engineers and scientists began to develop ways of correcting the problems by placing iron or magnetic objects near the binnacle.
In 1854, a new type of binnacle was patented by Liverpudlian maritime inventor John Gray. His brainwave was to incorporate into the pillar adjustable correcting magnets on screws or racks and pinions.
This system was further developed in the 1880s by Lord Kelvin, the mathematician, physicist and engineer best known for his work on thermodynamics – absolute temperatures are stated in units of kelvin in his honour.
His work on the maritime compass made it massively more reliable – he patented a system in which the compass incorporated two spherical compensating magnets, one either side of the binnacle, often painted red and green, and colloquially known as ‘Kelvin’s balls’ or ‘navigator’s balls’.
A Johannsen & Co was founded in 1859 by the Danish-born Asmus Johannsen; in 1862, he brought his nephew (or possibly cousin, depending on which source you believe), Christian Carl Lorenzen, into the company. The company supplied chronometers – marine timekeepers – and other instruments to the Admiralty, the Indian government and to the navies of Italy, Spain, Portugal, and the States.
Johannsen was a regular participant in the Greenwich trials – annual events instituted by the Admiralty in the early 19th century to determine which were the best chronometers for use by the Navy – and in 1916 he took two first places at the event.