Electric egg ('aurora'?)

This apparatus owes its name to the egg-shaped glass tube. In this tube two electrodes are mounted hermetically, the upper electrode being movable. Because the lower electrode is earthed, connecting to a current supply causes a spark to jump from the upper to the lower electrode. The maximum distance a spark can bridge between the electrodes depends mainly on the strength of current. However, if the glass egg is pumped nearly vacuum, something strange happens with the spark. Instead of a short spark discharge, the air between the electrodes starts to glow. This causes violetlike bands of light from top to bottom. The rarer the air, the wider these bands of light become. The electric egg and similar instruments were used to study and explain atmospheric phenomena, such as the northern lights. Because of the beautiful light effects the electric egg was also very popular as a demonstration apparatus for a broad public.