A high-voltage electric current, passed through a gas at a pressure much lower than that of the atmosphere, produces a fascinating glow as the gas is ionized. In 1855, Heinrich Geisler (1815-1879), a glasblower in Bonn, devised a new pump that produced a much better vacuum than ever before. He used the pump to evacuate intricately shaped glass tubes in order to demonstrate this colourful electrical effect. These tubes became known as Geissler tubes.
© Science Museum / Science & Society Picture Library