Although magnetic observations had been sporadically made since Flamsteed’s time, it wasn’t until 1817 that a permanent observatory was set up at Greenwich. The Magnetic Observatory moved to Abinger in Surrey in the early 1920s due to potential interference from the nearby railways which were about to be electrified. It was operational there from 1924 until 1957 when it moved to Hartland in Devon.
In 1816, the Admiralty asked the Astronomer Royal, John Pond, to make regular observations of magnetic variation (declination). A Magnetic House was put up during April and May 1817 (RGO6/1/54), and observations commenced in 1818. Airy records in his published plans (1847 & 1863) that it was built in the Lower Garden. This was enclosed from the park in two stages, the northern half in 1814, and the southern in 1837. It consisted of a deep semicircular hollow in the hillside with a more or less level bottom. In 1817, the level part was in use as a kitchen garden and it was perhaps for this reason, that Pond made the unwise decision to place the observatory on the east side on the slope of the hill. The foundations rapidly gave way, causing the building to become so dangerous, that the instruments were removed. It was demolished in 1824 and magnetic observations ceased until a new observatory was established by Airy in 1838.
In 1829, a project for the simultaneous observation of magnetic declination on about six pre-selected days each year at about 20 sites across Europe and the Russian Empire was initiated by Alexander von Humboldt in collaboration with Carl Gauss. The aim was to determine the extent and simultaneity of the disturbances. In 1836, Humboldt via the auspices of the Royal Society (who brought in the recently appointed Astronomer Royal George Airy to advise), managed to get the work extend across the British Empire.
Recognising the importance of the magnetic work, Airy persuaded the Admiralty to fund the setting up and staffing of a magnetic observatory at Greenwich. The existing site was cramped, and Airy’s proposal to build in what is now called the Meridian Garden (and unbeknown to him at the time) above Flamsteed’s well, was rejected by the Visitors. Instead, it was decided to seek to enclose more ground from the Park. By June 1836, Airy was in correspondence with the Commissioner of Woods and Forests to secure it. An extension southwards was fenced off in the spring of 1837 (Work 16/126). Bounded to the east by Blackheath Avenue in the Park, this new enclosure had the additional advantage for Airy of both extending and increasing the privacy of the Lower Garden, which up until then had always been overlooked.
When the building of the magnetic observatory was sanctioned, it was proposed that the observations should continue for three years, but in 1842, it was determined that a further three years should be funded. This period expired in December 1845, and upon urgent representation of the Geographical Society, the Treasury again consented to extend the time. It took until 1884 before the Magnetic Assistant was employed on the same terms and conditions as the Astronomical ones.
In 1846, a porch was added to the main building. On 23 April 1853 there was a small outbreak of fire which fortunately ‘did little mischief’. In 1862, the building was extended northwards and the former Dip House and Deflexion House, were taken down as they were in the way of a range of seven wooden rooms (usually referred to as offices) that were about to be built running parallel to the southern boundary. Of these, the most easterly (office 1) was used as an experiment-room, whilst the most westerly (office 7) contained the Deflexion-apparatus and the Dip-Instrument. The remainder were appropriated to various operations of photography – photographic registration having been first introduced in 1847, in what was one of the earliest scientific uses of photography. On the south side of the range, at a small distance, there was a long desk that was used for making contact prints from the photographic plates, which in those days, still required a lengthy exposure in daylight. Beyond this a piece of ground a few feet in breadth was left vacant. In 1872/3 when the Observatory obtained its first photoheliograph for solar observation a small building with a rotatory dome was erected on the south side of and communicating with Magnetic Office 3, Magnetic Office 2 being altered for use as a Chemical Room.
Further changes were made in 1864, when a basement to the Magnet House was excavated in an attempt to provide an environment with a more stable temperature. Its walls were made of specially selected bricks, which were almost free of magnetism. The deep ground thermometers of the Meteorological Observatory had already shown that not too far below the surface, the ground temperature is extremely stable. Christie took further advantage of this when a drain was being laid for a new sink in the basement. He explained what had been done in his 1886 report to the visitors:
‘A line of 9-inch pipes about 155 feet in length has been laid underground for the ventilation of the Magnet basement, the depth below the surface varying from 5 feet at the lower end in the South Ground to 11 feet 6 inches at the entrance to the basement. Thus the air which is admitted (by means of two branch pipes) into the east and west arms of the basement is warmed in winter and cooled in summer by passage through a considerable extent of soil at nearly constant temperature.’
What Christie had built was an early form of heat pump. Interestingly, it was Lord Kelvin, a member of the Board of Visitors and later its chairman, who developed the theory of the heat pump – something he is said to have done by 1852. Rather regrettably, those responsible for the archaeological survey that took place while the excavations for the new planetarium were being carried out in 2004, were poorly briefed and had not done their homework. They were unaware of the pipes, which were duly bulldozed without being recorded – or probably even noticed. A similar fate awaited the bricks of the basement. Their presence was noted, but none were taken away to test just how magnetic they were.
When the Lassell Dome was built in 1883, it was erected immediately to the south of, and connecting to Magnetic Office 7, in which the dip and deflexion instruments were located. These instruments were moved to the new library. The new dome and telescope contained copious amounts of iron, whose effects on the magnetic instruments were much exacerbated when construction of the South Building with its steel frame began in 1891. The building of the Altazimuth Pavilion nearby in 1894-5 exacerbated the problem. Although Christie had hoped otherwise, it was soon apparent that certain of the magnetic instruments would need to be moved to a new location. When construction of the north wing of the South Building began at the end of 1894, the Magnetic Offices had to be removed. Rather than demolish them, they were literally picked up, turned through 90° and dumped near the western boundary in such a way that Magnetic Office number 7 which had been the most westerly, was now the most northerly.
It was in 1894, that Christie turned his attention to relocating part of the magnetic observatory on a new site in the park on the other side of Blackheath Avenue. As things turned out, things did not go entirely to plan. The new site was required to be magnetically neutral, and it took until 1897 to identify a suitable spot. Located some 400 metres to the East of Flamsteed House, it was considerably further from the main site than Christie had initially anticipated. The site, about 0.9 acres in extent, was originally known as the Magnetic Enclosure, but is now referred to as the Christie Enclosure. Having got his site, Christie was then beset by delays in the building of the new magnetic pavilion. This was completed at the end of summer 1898 and the bulk of the instruments transferred across. In March 1914, a second building was completed to house a set of modern magnetic instruments. Constructed by the Works Department of the Admiralty, it consisted of a thickly-walled outer room containing an inner room, well insulated by a considerable air-space. To maintain the constancy of the temperature, electric heaters, controlled by a thermostat, were installed. It was known as the Magnetograph Building
Meanwhile, back on the main site, the relocated Magnetic Offices were used initially as regular offices. Following the completion of the South Building, they were no longer required for this purpose and by 1905, appear to have found a new use as stables. With the outbreak of war in 1914, Dyson had a range-finder fitted onto the roof of the old Magnetic Building. The building was demolished in 1918, some of its parts being reused to construct a small hut in the Christie Enclosure for the electrometer, which was transferred at this time.
As early as 1890, interference began to be detected from the South London Electric railway some 4½ miles away. By 1893, a clause had been agreed for insertion in future parliamentary railway or tramway bills authorising electrical power, in order to protect the magnetic observatories at Greenwich and Kew (established in 1857) along with various other government scientific establishments in London. As time went on and more train and tram lines became electrified the problem was kept under control. The First World War came and went and was followed by a rationalisation of the railways and with it a greater degree of standardisation. To this end the Ministry of Transport set up an ‘Electrification of Railways Advisory Committee’ chaired by Sir Alexander Kennedy. The outlook for the Magnetic Observatory at Greenwich soon began to look bleak, so much so, that on 5 October 1921, a conference was convened at the request of the Admiralty to discuss the desirability & practicalities of relocating the magnetic observatory. A site at Abinger Bottom near Leith Hill, some 26 miles distant from Greenwich, was formally acquired in February 1924. Absolute observations began on 24 March, with the site becoming fully operational on 1 April 1925. Meanwhile, back near Greenwich, the first electric trains began to run on 28 February 1926 with a full service from 6 June. At this point, all regular observations at Greenwich with the absolute magnetic instruments ceased. More about the events leading up to the move wasy from Greenwich can be read in the section on Abinger.
By the 1940s, the former Magnetic Offices were in use as follows Offices 1 & 2 (the most southerly) as garages, Office 3 as an empty case store, Office 4 as a paint store and offices 5 to 7 as stables. They were badly damaged by blast in the Second World War and were demolished some time around 1959/60. Back in the Christie Enclosure, the Magnetic Pavilion was demolished in 1932 and the Magnetograph Building became known as the Meteorological Recording Building. It was demolished in 1959 when the Enclosure was cleared of buildings and returned to the Park.