Describes an instrument for the measurement of air infiltration into buildings. The instrument indicates the concentration of a tracer gas in the building, by sensing the thermal conductivity of the air-tracer gas mixture.< The instrument, a Katharometer, has not been extensively used because of inherent difficulties. This thesis examines the dificulties. An analysis of the theoretical basis of the instrument is made and the problems created by changes of relative humidity, pressure and temperature are discussed and solutions are suggested. With control of these factors, satisfactory results were not obtained until a voltage control was used. A satisfactory automatic voltage control was developed that has been sucessfully used in air infiltration measurements.