Weights and Measures Act 1985

Part III U.K. Description of United Kingdom primary standard of the metre

The British copy of the prototype metre, being a bar about 102 centimetres long with a cross-section of modified X-form and made of platinum-iridium alloy (90 per cent. platinum, 10 per cent. iridium), bearing at one end the markings “0°C & 20°C”, “A.16 SIP GENEVE 1956” and (on the cross-section) “1” and at the other end the markings “B.16” and (on the cross-section) “2”, and having engraved on the exposed neutral plane—

(a) near each end and also at the centre, two parallel longitudinal lines about 0.12 millimetre apart;

(b) near the end marked “1” and at the centre, one transverse line; and

(c) near the end marked “2”, two transverse lines about 0.17 millimetre apart,

measurement being made of the mean interval between the portions of the most widely separated transverse lines which are between the respective longitudinal lines when the bar is at the temperature of 0° Celsius, is subjected to an atmospheric pressure of 1013.250 millibars, and is supported on two rollers at least one centimetre in diameter placed symmetrically 571 millimetres apart in the same horizontal plane.