A description is given of a new 1 m comparator having two heat insulated tanks designed to contain line standards of length and to bring them in turn under a pair of micrometer microscopes rigidly fixed at a distance apart equal to the nominal length of the standards to be compared. The instrument can be used (a) to compare the lengths of two standards placed side by side in the same tank and at nominally the same temperature or (b) to determine the coefficients of thermal expansion of the materials of which reference standards of length are usually made, by having one standard in one tank and maintaining its temperature constant whilst the temperature of the standard in the other tank is varied. The instrument embodies a number of improvements designed to eliminate all foreseen sources of systematic errors. Results are given which indicate that line standards of length can be measured on the comparator with an accuracy approaching one part in ten millions and that, over a range of about 30° C, mean coefficients of expansion of the order of 10^-5 per 1° C can be determined with an accuracy of about one-tenth of 1 per cent.