SMALL models of the Earth, called 'terrellas', have been used in science since the late sixteenth century to investigate and demonstrate magnetic, electrostatic, and electromagnetic phenomena.
The term terrella is first found in William Gilbert's book De Magnete, published in 1600. Gilbert described the Earth as a large magnet and made small spheres out of lodestones to demonstrate the effects of terrestrial magnetism. Later in the century, around 1660, Otto von Guericke made a small sphere from sulphur, which could attract dust and small particles by static electricity, in order to model the attractive effects of the Earth's gravitation.
In the mid-nineteenth century, when electric and magnetic phenomena had become better understood, several scientists continued to use electromagnetic terrellas for experimental and demonstration purposes.
More recently still, at the beginning of this century, the Norwegian physicist Kristian Birkeland (1867-1917) began his own study based in this long tradition of work with terrellas. Between 1896 and 1913 he pursued a series of gas-discharge experiments in his laboratory at the University of Christiania (modern Oslo), directed primarily at reproducing in the laboratory the effects of the Aurora Borealis.
From 1900 Birkeland performed all his experiments using an electromagnetic terrella as one of the electrodes in a gas-discharge apparatus, to create an artificial Aurora around the poles of the terrella, replicating the effects of the solar wind on the magnetic Earth. He also attempted to simulate further cosmic phenomena, such as the Sun's corona, sunspots, and the rings of Saturn, using other small metal spheres.
Throughout his experimental career, Birkeland made use of a whole range of terrellas, varying from 2·5 cm to 36 cm in diameter. Each consisted of an electromagnetic coil inside a sphere of brass or aluminium. In experiments where the visual effects on the terrella were small, fluorescent paint was often used on the surface to amplify the phenomena.
Birkeland's largest experiment was carried out in 1913 in a large vacuum chamber of 1,000 litres capacity with terrellas of 24 and 36 cm in diameter. This apparatus became very well known for its ability to recreate Aurora effects and in Norway its fame is such that it is depicted on the Norwegian 200 kroner banknote.
The chamber of the 1913 experiment was constructed much like an aquarium, with sides fabricated in glass, the top and bottom in brass plate, and the whole structure kept together by four corner posts. Each original glass side was a flat, solid glass block, 4·7 cm thick, 100 cm in width, and 70 cm in height. In the top of the chamber was an oval inspection hatch, large enough for an assistant to pass though in order to carry out repair and maintenance work.
In use, the chamber had to be kept evacuated by strong pumps. To keep it air-tight all gaps and potential paths for leakage were covered with picein - a black, tar-like substance widely used for the purpose in laboratories of the time.
During experiments, the terrella was suspended in the middle of the chamber from the upper plate. Gas discharges were initiated between its surface and a disc-shaped electrode at one corner of the chamber. The effect of the Earth's magnetic field on these discharges could then be simulated by energizing the magnetic coil in the terrella, the coil being tilted slightly to mimic the slight variation that exists between the Earth's magnetic and geographic poles.
The original chamber and a 24 cm terrella survive to the present day and are now kept in the Auroral Observatory at the University of Tromsų, Norway. Over the years the chamber had fallen into a poor state of repair. The glass windows were partly broken and the original sealing agent had been removed. Pump connections and electrodes were missing, and electric insulators had crumbled away.
In 1995 it was decided to restore the whole of Birkeland's experimental apparatus to working order. Although several missing parts would necessarily have to be replaced, the chamber and the terrellas are so important as objects in the history of Norwegian science that it was decided that no changes to the original components could be allowed during the restoration work.
With such constrictions, a range of questions and considerations had to be taken into account in planning the programme of restoration. Compromises had to be made based on safety, budget, maintenance possibilities, and the strong desire to restore the apparatus to its original appearance as closely as possible.
One of the most important considerations was that of the force of atmospheric pressure on the walls of the chamber. When the chamber is evacuated, each square centimetre of its outer surface is subject to an air pressure of approximately one kilogram. This results in a pressure of 10,000 kg (10 tonnes) on the top and on the bottom plate, and of 7,000 kg on each window. If a window should break during a demonstration due to the external pressure of 7 tonnes, the implosion could throw pieces of glass into the room, causing serious injury to onlookers.
The new windows had to be absolutely safe. They were made of laminated glass, a little thicker than the original, but mounted in such a way that their outside surfaces were flush with the frame of the chamber. Because of the air pressure, the top and bottom plates had originally been strengthened with brass beams fastened by copper rivets. As part of the reconstruction process, calculations were made of the loads on the windows, the plates, the beams and the rivets, to confirm that dangerous implosions could not occur.
All the joints in the chamber had originally been sealed up with picein, including the glass blocks where they met the corner posts. With a total length of picein-sealed joints of some 55 metres (including the circumference of each of the 250 rivets) there were rich opportunities for leaks to occur.
To deal with the potential leak issue, it was decided to make a compromise and to use rubber gaskets as seals around the oval inspection hatch and between the windows, while a sealing agent close to the original type was used to cover the rivets and all other gaps. The use of this sealing strategy made maintenance feasible as well as giving the chamber the feel of the original in both look and operation.
The whole sealing operation and leak-tightening process proved to be long and difficult. The sealing agent was applied with a hot-air gun with the windows in place and the vacuum pump in action. The edges around all the rivets were first covered, before the fissures between the plates and beams. This was especially difficult on the bottom of the chamber, with the tendency of the warm tar to drip.
The sealing work took two weeks to complete before all the major leaks were sealed or reduced to such a state that a pressure low enough for obtaining gas discharges could be maintained. The result was a chamber that looked increasingly like its appearance in old photographs, liberally covered with black sealing agent.
Many of the records of Birkeland's vacuum and other laboratory methods have disappeared and he left few technical details of how the chamber should be operated. The actual process of restoring the experiment using the old components revealed much that could not be inferred from the published documentation and shed light on sources well known but not fully understood. Practical skills and laboratory techniques taken for granted at the beginning of the century had to be learnt again from scratch. More than anything else, the restoration work showed that the strength of the glass and the leakage problems must have been some of the main problems more than eighty years ago.
The main issue for the restorers, rather than the exact replication of Birkeland's original experiments, was to be able to recreate the effects of the Aurora Borealis for demonstration purposes. Considerable success was attained in this, and after further work displays simulating Saturn's rings, sunspots and the corona of the Sun were also successfully carried out.
Birkeland often expressed a considerable amount of satisfaction and joy over seeing the beautiful and colourful discharge phenomena around his terrellas; emotions little diminished in the feelings of the restorers when the first violet glow formed at the corner electrode and flickering rings of coloured light appeared around the north and south poles of the terrella in the refurbished chamber.
In addition to their use in investigative scientific studies, Birkeland himself had often run his terrella experiments purely for demonstration purposes. After his death the original apparatus continued to be used on occasions in public lectures to display the artificial Aurora Borealis and other phenomena.
The newly-restored chamber has again become a popular object of attention, for visitors to the Aurora Observatory at Tromsų - a modern role not in fact very far from the original one for which it was designed. Today, the modern viewer standing in front of the terrella can hardly be disappointed by the brilliant and beautiful display of light around the Earth in miniature.
Terje Brundtland, M. Sc., '97-'98