A discharge tube, or a gas-filled tube is an arrangement of electrodes in an insulated container, containing a gas. Very often, glass or ceramics are used as insulation. When there is enough charge on these electrodes, there will be an electric discharge, which will produce light. In the 19th century, Heinrich Geißler developed the Geissler tube,and Philipp Lenard did experiments that led to the development of the vacuum pump. Both technologies were used to develop the discharge tube. The most common kind of discharge tube is the gas discharge lamp.