We describe a dilatometer which is an improved version of the one previously used in this laboratory. It consists of a doubly twisted strip of beryllium copper with a mirror attached to its central region. This system is immersed in liquid helium II. Dilations of the sample are sensed by this system via a thin diaphragm, causing rotations of the mirror, which is detected by an external optical lever. The system has a sensitivity to relative length changes &Dgr;l/lof 10−11. Copper has been used as the test material to evaluate the performance of the instrument. It has been possible to make direct measurements of &agr;, the coefficient of linear thermal expansion, down to 2 K for &Dgr;T=0.2 deg. These results yield &agr;=1.3×10−10T+2.7×10−11T3deg−1. The ratio of these terms is much more accurately known than the absolute values because of uncertainty in the absolute calibration. The linear term leads to an electronic Gru¨neisen &ggr; of 0.57. This value is compared with recent theoretical and experimental values.