of vertical axis positioned at the centre of the cup shaped support 3 the two horizontal nozzles 14 terminate at an upperly open cavity 18 defined by a rim emerging from the sheet metal 6 of the cooking hob specifically this rim which forms all the side walls of the cavity 18 comprises a lesser projecting portion 20 of the cavity wall to which the nozzles 14 are applied and a greater projecting portion 22 involving the other three walls two parallel venturi conduits 24 of horizontal axis extend from that wall of the cavity 18 opposite the wall to which the nozzles 14 are applied these venturis extend from the cavity 18 they are spaced apart and after passing through virtually the entire cup shaped support 3 open into an arcuate channel 26 which extends as two branches themselves extending upperly into two vertical channels 28 which pass through the sheet metal 6 of the hob and are each provided with a rim 30 projecting upwards to an extent equal to the rim 22 of the cavity 18 the two horizontal venturis 24 extend spaced apart to define a space within which the nozzle 16 of vertical axis is positioned it is housed in a cavity 32 defined by side walls which emerge via their upper rim from the sheet metal 6 of the hob this upper rim comprises a portion 34 joined to the greater projecting rim 22 of the cavity 18 and having the same height thereas and a portion 36 more distant from the cavity 18 and depressed to an.