The generator takes the form of a thermal tube contg. a housing (1) with a wick (2) in it. Part (3) of the housing, the vaporisation zone, is heated by heater (4) and part, the condensation zone, is cooled by a system (6) e.g. by circulating water or air. Between the two zones (3,5) is an `adiabatic' zone (7) where the unit (8) for tapping off the vapour to the consumer (the thermal-emission transducer) is located.    The wick (2) is a return channel for the condensed working body from zone (5) to zone (3). A filter (9) for filtering off the oxides of the working body is located at the start of the return channel. For caesium the filter could be a pressed tube of thin (20-30 micron) fibres of stainless steel.    Once assembled, the vapour generator is filled with caesium by vacuum distillation with condensation of the vapour in the vaporisation zone (3). Zone (3) is then heated and vaporisation occurs from the surface (11) of the liq. working body, part of the vapour passing via unit (8) to the consumer and part via the vapour channel inside the housing (1) to the condensation zone (5) which is cooled by system (6). The vapour condenses and returns via the return channel (2) to the vaporisation zone (3).    When the consumer is in operation gases including oxygen are released and pass by diffusion via unit (8) into the vapour generator. The gases are carried away by the vapour flow into the zone (5) where the vapour and its oxides are condensed and the uncondensed gases are removed via unit (10) into the evacuation (or gas collection) system. The condensed working body and its oxides go into the return channel (2), pass via filter (9) where the oxides are retained and back to zone (3).    USE/ADVANTAGE -   For use in laboratory research and reactor testing of thermal-emission electricity-generating channels with sources of working-bodies, primarily alkali metal vapour e.g. in the design and construction of caesium vapour sources for thermal emission transducers. The efficiency and lifetime of the thermal emission transducer is increased by making provision for purifying the circulating working body of its oxides.