Helical pin gas discharge tube, component of carbon dioxide laser, made at the University of Hull, 1969. The tube would contain a gas mixture of 80% helium, 10% carbon dioxide and 10% nitrogen at atmospheric pressure. To initiate laser action, it is necessary to produce a homogeneous glow electrical discharge along the axis of the tube. This is attained by "summing" the individual spray discharges between opposite pins of the twin helix. The e.h.t. helix pins are ballasted by resistors.