Halley and the 'Paramour'

A page from De MagnetaA page from De Magneta by William Gilbert The year 2000 was exactly 400 years since William Gilbert (1540–1603), physician to Queen Elizabeth I, published the first scholarly work devoted to magnetism, in which he suggests that the Earth itself could be regarded as a giant magnet. It was written in Latin and entitled De Magnete.

It was also 300 years since Edmond Halley, the second Astronomer Royal, completed two voyages on the ship Paramour to investigate the variation between true and magnetic north and the possibility of using the difference to find longitude at sea.

To celebrate these anniversaries, a small exhibition ran in the Obervatory's Meridian Building. This page presents a few of the objects from the exhibition and some of the history of the compass as a navigational instrument.

Lodestone

From the 12th century in Europe, lodestone, or magnetite (Fe304), a naturally occurring magnetic ore, was used to magnetise mariners' compass needles. On longer voyages it was necessary to carry a piece of lodestone on board to remagnetise the needles. Until the late 18th century, compass needles were made of soft iron and so would lose magnetism relatively quickly.

The 15th and 16th centuries saw an increase in exploration and international trade and as a result the compass became more important as mariners travelled further afield. It was especially important for oceanic voyages in cloudy weather.

De Magnete

William Gilbert (1540–1603) was primarily a physician, a fellow of the Royal College of Physicians from 1573 and also appointed physician to the Queen. He was a scientist, or natural philosopher, to use the historical term, in his spare time. His greatest work, De Magnete, came out in 1600. It begins with a summary of all that was known about magnetism at that time, then gives an account of his own investigations into magnetism complete with experiments and diagrams.

It is in this book that he relates the attraction between individual magnets, to the attraction between magnets and the Earth's poles and from this concludes that the Earth can be viewed as a giant magnet. Gilbert first coined the name terrella, meaning 'little Earth' to describe a small sphere of magnetite or lodestone, used by scholars to study magnetism.

Edmond Halley

Edmond Halley by Sir Godfrey KnellerEdmond Halley, second Astronomer Royal by Sir Godfrey Kneller, circa 1721 Almost 100 years after Gilbert published his famous book, Edmond Halley (1656–1742), already an established astronomer and natural philosopher, set sail in command of the ship Paramour to investigate a possible solution to the longitude problem. By this stage in his career, Halley, now aged 42, had already mapped the stars of the southern hemisphere from the island of St Helena, worked for Flamsteed at the Royal Observatory and been instrumental in the publication of Isaac Newton's Prinicipia.

In 1698, in the hope that Halley might be able to solve the longitude problem, the Royal Navy made him a captain and he set sail on the Paramour to chart the variation of the Earth's magnetic field in the western and eastern hemispheres. As it turns out, the variations fluctuate with time and cannot be used to find longitude at sea.

The compass

Mariner's compassMariner's compass made by A. H. van dan Meulan, c. 1720, Rotterdam, Netherlands Though not useful in determining longitude, the compass continued to be important in navigation. In design it changed little between around 1550 and 1750. The azimuth compass was a step up from the standard mariners compass. These compasses incorporated a means of aligning the compass with a celestial body such as the Sun or the Pole star. The reading from this alignment would then give another reading for north which could be compared with that given by the compass needle thus allowing the variation to be easily read.

In China compasses were used not only by mariners for navigation, but by geomancers to align buildings in order, so they believed, to maximise the good fortune of the occupants.

The binnacle

Binnacle and compassBinnacle and compass, made by Sir William Thomson and James White, 1876. By the 19th century the variation between true and magnetic north was well understood. From 1858 onwards British Admiralty charts were produced at regular intervals to be used by mariners so that they might be aware of current variations. However a new problem was now facing mariners with regard to magnetism.

The growing use of iron in the construction of ships, from the late 18th century onwards, affected the accuracy of the compass. Local attraction by the iron caused compass needles to deviate from magnetic north in unpredictable ways.

George Biddell Airy, the seventh Astronomer Royal, was an important figure in the development of a solution. In 1839 he published a paper in the Philosophical Transactions of the Royal Society discussing the science behind the problem. His solution of introducing corrector magnets only achieved partial success.

Expanding on Airy's idea, Scottish mathematician and physicist Sir William Thompson (later Lord Kelvin) of Glasgow University developed a new compass housing to overcome this problem in 1876. His patented binnacle incorporated corrector magnets and soft iron spheres to counteract the effects of the ship's iron. He also devised a much lighter compass card with two or more short parallel needles, which was less subject to oscillation and wear than the cards previously used.