226,790. Metropolitan-Vickers Electrical Co., Ltd., (Assignees of Styer, C. A.). Dec. 29, 1923, [Convention date]. High potential switches. -In tanks for oil-immersed electrical apparatus, e.g. transformers and circuit-breakers, an inert gas is maintained in the space above the oil by means of an electricallvheated carbonaceous material, through which the gas in the space above the oil, and the air entering the tank, passes. In one arrangement, Figs. 1 and 2, a carbonaceous material is placed in a container 7 mounted in the cover 6 of the tank 1. The lower part of the container is provided with heat-insulating-material 11, above which is a hopper 21 containing a supply of sugar carbon, or other carbonaceous material. The gas within the tank circulates through passages 12, 17, in the former of which is placed a piece of wire gauze 13, which supports a layer 16 of carbonaceous material surrounding electric heating elements 14, 15. A tube 19 communicates with the outside atmosphere. In a modification, Fig. 5, the carbonaceous material occupies the space between concentric gauzes 32, 33, a tube 42- communicating with the outside atmosphere. In a simpler construction, a layer of gauze extends across the whole width of the container and supports the carbonaceous material.