When human skin is stimulated with thermal radiation so that sensations of warmth are evoked, it is most sensitive to radiation with a peak wave-length of 35,u, according to . However, during an investigation of the effects of infra-red radiation on thermal comfort (Chrenko, 1953), discomfort was experienced when the temperature of the source of radiation was only 650 C, so that the skin was more sensitive to radiation with a peak wave-length longer than 3x5 It.Further experiments have been made to determine quantitative relations between the peak wave-length, i.e. the wave-length of maximum energy of thermal radiation, skin temperature, and the sensory threshold for warmth.
METHODS
ApparatusThe thermal stimulus was radiation emitted from a black body or full radiator. The inner core of the black body was a 30 cm diameter hollow sphere of ceramic material, with an aperture circular in shape and 10 cm in diameter, fitted to a hollow cylinder of the same diameter and length. Bricks of refractory material were used as thermal lagging, which covered the sphere so that the furnace assumed the shape of a cube with sides 60 cm in length.The furnace was mounted so that the longitudinal axis of the cylinder leading to the hollow sphere was horizontal. The furnace was electrically heated and its temperature was altered by varying the voltage of the electrical input. The maximum electrical loading of the furnace was 3 kW. The arrangements are shown in Fig. 1. Details of the thermocouple used to measure the furnace temperature are shown in Fig. 2. From the furnace temperature, the peak wave-length was calculated from Wien's equation, tf = (2892/A,, .) -273, where Amax. is the peak wave-length (,u), and tf is the temperature of the source of radiation (O C). The intensity of the radiation emitted by the furnace was measured with a calibrated thermopile with an output of 64-5 microvolts/milliwatt/square centimetre of incident radiant energy.A large water-cooled screen, with a circular aperture and shutter, was mounted vertically in front of the mouth of the furnace. The shutter was controlled by an electronic timing 1 Physiol. 173