Fire Safety Science Digital Archive

Although the differential expansion of metals has been exploited in the measurement of temperature this property does not appear to have been exploited in the measurement of radiation. A radiometer may be constructed by fixing two bimetallic strips in an apperture in a water-cooled screen (Fig. 1). As the strips are heated by radiation they deflect, their deflections may be arranged to be opposite senses so as to twist a yoke carrying a mirror, or the deflection may be in the same sense and the deflection measured by a reading telescope (Fig. 2). After the deflection has been measured the instrument may be calibrated by substituting electrical heating for the radiant heating until the same deflection is given.