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An instrument is described which measures separately the convective and radiative components of heat transfer within a large flame. Its principle of operation is that the convective transfer to smooth surfaces, whether polished or blackened, exposed to a given flame is the same but heat transfer by radiation is proportional to surface absorptivity. Two gold-plated copper segments, one blackened on its receiving surface, are exposed simultaneously for a few seconds within a flame, and radiation and convection components are obtained from the rates of temperature rise of the segments and separately measured surface absorptivities. Although cooling corrections are necessary they can be simply made since the cooling rate varies linearly with temperature excess. The instrument can be used satisfactorily in flames of alcohol, town gas and wood.