The two commonly occurring boundary conditions in heat conduction problems of known heat transfer and radiation have been represented on a resistance-capacitance electrical analogue for which the time of operation is about one minute. If the coefficient of heat transfer or the forcing temperature varies with time, the effect can be included by using uniselector switches. The simulation of radiation is achieved using a semiconductor, Metrosil or Atmite, in conjunction with a constant current supply based on a pentode. Particular attention is drawn to a method of altering the conductance constant of a semiconductor by choosing an appropriate voltage scaling factor. Three examples illustrate the principles; the dipping of a bar into a cooling medium, the effect of thermal shock on a turbine blade and the radiant heating of a thick hollow cylinder.