Special valves have been constructed to meet the constant volume requirements of a gas thermometry program. With a total volume inside the valve block less than 20 mm3, and that which is a part of the gas thermometer about 3 mm3, no change in excess of 0.005 mm3is expected. This is achieved by extending the valve stem past the seat through a seal on the opposite side, so that the volume of stem pulled into the gas chamber is equal to the volume withdrawn when the valve is opened.