GB1165724A - Improvements in or relating to High Pressure Metal Vapour Electric Discharge Lamps - Google Patents

Abstract

1,165,724. Discharge lamps. GENERAL ELECTRIC & ENGLISH ELECTRIC COMPANIES Ltd. 2 Jan., 1967 [12 Jan., 1966], No. 1467/66. Heading H1D. In a double-walled high-pressure metal vapour electric discharge lamp incorporating, between the discharge tube and outer envelope, a thermal starter switch with its contacts shunted across the lamp electrodes and which comprises a snap-acting bimetallic element carrying a movable contact which is normally closed on to a fixed contact andaresistanceheating element which is used to heat the bimetallic element sufficiently to cause the contacts to snap open to initiate the discharge, the arrangement of the switch in the outer envelope is such that the bimetallic element remains sufficiently heated during running up and normal operation of the lamp that the switch contacts remain open, and, in the event of a sudden extinction of the lamp discharge, the switch contacts are delayed from reclosing until the tube has cooled to a temperature at which the filling pressure has decreased to a value when operation of the starter switch can restart the discharge. The resistance heating element may comprise a heating coil disposed adjacent the bimetallic element or may be provided by the bimetallic element itself. The bimetallic element is formed by a preset slightly dished plate to provide the snap action. The discharge tube within the outer envelope has a filling of rare gas and metal vapour at at least 100 mm. of Hg pressure when operating. In an example the filling comprises argon to several mm. of Hg pressure and mercury and sodium, thallium and indium iodides to about 5 atmospheres operating pressure.