A method is described for producing a constant temperature interval across a liquid film, using a semiconductor heat pump. Current pulses pass through the heat pump in each direction (period of complete cycle normally about 3 seconds), their exact magnitude and duration being controlled by a pair of thermistors placed on either side of the liquid film. By this means, temperature differences of up to 3 degC were held indefinitely across a 1 mm thick film, with a variation of ±0.003 degC.