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 continuestion 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.