Silicon cell for the precise measurement of thermal expansion at low temperatures: Results for Cu and NaF

A three terminal capacitance dilatometer for low temperatures is reported in which the expansivity of a sample is compared to that of silicon, a material of very small expansivity at low temperatures. The thermal expansion of a standard copper sample is measured down to 3 K and the results compared with other authors. Values for the linear thermal expansion coefficient of a pure NaF single crystal below 5 K are reported for the first time.