The apparatus described was developed to measure length changes in germanium due to electron irradiation and subsequent isochronal annealing at low temperatures. Similar techniques would be applicable to other experiments. Length changes of the sample produced changes in spacing of a system of three‐terminal capacitors. These capacitors were measured by an ac ratio bridge capable of detecting changes in capacitance of 4 parts in 107. The irradiated portion of the sample, the surrounding framework, and one plate of each of the parallel plate capacitors were all part of the same monolithic germanium single crystal. A system of six capacitors was used to eliminate extraneous effects. The sensitivity and long term stability was on the order of 0.1 Å which corresponds to 2 parts in 109for an irradiated sample length of ½ cm.