Date: 3rd quarter 18th century

Description

The electrical or “philosophical” egg became in the nineteenth century a classical instrument to study the effects of electrical discharges in rarefied gases. It consists of an ellipsoidal glass vessel with two electrodes, each one ending in a brass sphere. Outside the egg, the upper electrode terminates in a brass ring provided with a screw terminal.  The lower electrode can be moved upwards and downwards thanks to a rack-and-pinion mechanism moved by a lateral screw. The instrument could be fixed to a pneumatic machine, in order to rarefy the gas it contained. A cock placed near the base served to open and close the communication between the egg and the outside. The handle of the cock is also equipped with a screw terminal.

By connecting the two electrodes to an electrostatic generator or a Ruhmkorff coil, one could observe the luminous effects of electrical discharges, which change according to the internal pressure. For instance, as Ganot writes, “with a powerful electrical machine”, one could see “from one electrode to the other, a violet light, not very intense and continuous”. When some vapors were introduced, like the essential oil of  trementina for instance, one could observe “a series of zones alternatively bright and obscure”. Moreover, “most of the times, the light of the positive pole was red, and the light of the negative pole was violet. But the color changes with the vapor or with the gas introduced in the globe” (Ganot, 1870).

Materials and techniques: brass/glass

Dimensions: height 45 cm, diameter 15 cm

Keywords: electricity, electrostatics

University of Padua, Museum of the History of Physics

Cat. Number: 224

Exhibitions

• "Bagliori nel vuoto. Dall'uovo elettrico ai raggi X: electricity e pneumatica dal Seicento ad oggi", Padua, Botanical garden, 1 February-30 June 2004

Bibliography

• Auguste de La Rive, Traité d’électricité théorique et appliquée, 3 vols., Paris, 1854-1858
• Jules Jamin, Cours de physique de l’Ecole Polytechnique, 3 vols., Paris, 1858-1866
• Théodose du Moncel, Notice sur l’appareil d’induction életrique de Ruhmkorff, 4 ed., Paris, 1859; 5 ed., Paris, 1867
• Pierre Adolphe Daguin, Traité élémentaire de physique, 3 vols., 2nd ed. Paris, 1861
• Adolphe Ganot, Traité élémentaire de physique expérimentale et appliquée et de météorologie, 14 ed., Paris, 1870
• Rinaldo Ferrini , Elettricità e magnetismo, Milano, 1878
• Gustav Wiedemann , Die Lehre von der Elektricität, 4 vols., Braunschweig, 1882-1885
• Julien Lefèvre, Dictionnaire d’électricité et de magnétisme, Paris, 1891