The cooling of a surface when thermionic electrons are emitted has been measured by Richardson and others. The results obtained for tungsten and other metals are consistent with recent determinations of the work functions by thermionic and photoelectric methods. The purpose of the research, here described, was to determine the cooling of a surface when photoelectrons are emitted. In the determination of the cooling of a surface when photoelectrons are emitted, the small electron currents possible demand a very sensitive temperature measurement. A bolometer method similar to that used in the thermionic work was first tried but did not prove satisfactory; so this method was discarded, and thermocouples were used instead. Two similar photoelectric cells were illuminated with the same source of light. Each cell contained a small platinum plate coated with caesium, the temperature of which was measured with an Fe-Ni thermocouple. The thermocouple junctions of the two cells were connected in series with a galvanometer in such a manner that the e.m.f's of the two cells opposed each other. In this way it was possible to determine, with the galvanometer, the change in temperature when the photoelectric current was allowed to flow or was stopped in one of the cells. The photoelectric current was controlled by the potential of the anode.