Abstract
This paper reports on recent intercomparisons of the U.K. yard standards (formerly known as the imperial standard yard and its parliamentary copies) and their determination in terms of the international metre. It has been found that, in May 1963, the length of the `United Kingdom primary standard of the yard' (yard No 1) was equal to 0.914 396 9 metre and its length is decreasing at the rate of 1 part per million in 23 years. The yards Nos. 2, 3 and 5, of which the current values are also given, have shortened in recent years at about the same rate, while the (newer) yard No VI is shortening at about twice this rate. In relation to the yard unit of 0.9144 metre defined by the Weights and Measures Act 1963, which came into force in this respect on 31st January 1964, the error of the yard No 1 is -0.000 12 in., i.e. this standard is 1 part in 300 000 too short. The yard unit equal to 0.9144 metre has been the basis of length measurements made in British units at the National Physical Laboratory since July 1959.