This apparatus, created by the Geneva physicist Arthur Auguste De La Rive (1801-1873), who was also the Rector of the Geneva Academy, to study how the appearance of an electric spark is modified in rarefied gases with variations in pressure and the gas used.
In the traditional version it consists of an egg-shaped glass globe with very thick walls, which is closed both at top and bottom by brass rings through which brass rods terminating with a spherical electrode penetrate. The distance between the two electrodes can be varied by raising or lowering the upper rod, which slides through the airtight cap.
The bottom ring, which is attached to a tube with a cock, terminates on the wide circular base of the apparatus, which has a hole to receive it. The edge is perfectly flat so that it can be placed hermetically on the plate of a pneumatic machine. Once a vacuum has been created, the spark can pass between the electrodes once they have been connected to an electrostatic generator or to the terminals of a Ruhmkorff coil.
The name derives from the fact that when the globe contains air and the pressure is reduced to a few millimetres of mercury, the spark between the electrodes takes place in continuation and assumes the appearance of a luminous spheroid called the electric egg. The exemplar shown here has an overall height of 52 cm and differs from the one described in that the glass globe is spherical in shape (diameter 15 cm) and terminates at the top with a wide circular aperture (8.15 cm) with a well-flattened edge.
The support of the upper electrode lies on that edge by means of a brass disk, it too well flattened, which was covered with a thin layer of grease to ensure its being airtight.
This aperture allowed a few drops of certain liquids to be introduced and thus observe the sparking through their rarefied vapours.
Daguin (1878), T. III, p. 206
Drion - Fernet (1883), p. 411
Privat Deschanel (1869), p. 568
Ragozzino-Schettino (1985), p. 34