Faraday's egg. An egg-shaped glass flask made by Michael Faraday (1791-1867) to study the flow of electricity. Faraday was trying to discover whether matter was required for an electrical current to pass. He created a vacuum in the flask, then applied a voltage across the electrodes. He observed a bright yellow spark. By further experiments he discovered that the colour of the spark depended both on the type of metal in the electrode and the type of gas present in the flask. This was the principle that led to the development of spectroscopy. This object is preserved at the Royal Institution, London.