NPL Standard ohm

Standard mercury ohm

This is a mercury standard of resistance, prepared by the National Physical Laboratory to represent the value of the International ohm as defined by the Conference on Electrical Units and Standards held in London in 1908.

At this conference, the International ohm was defined as “the resistance offered to an unvarying electrical current by a column of mercury at the temperature of melting ice, 14.4521 gm. in mass, of a constant cross-sectional area, and of a length of 106.300cm”. The glass tubes used for standards were to be well annealed and straight; the bore was to be uniform, circular and of cross-sectional area approximately 1 sq. mm. End vessels, spherical in shape and about 4 cm in diameter, were to be fitted on to the tube to carry the connections for the current and potential terminals when resistance measurements were made. The formula to be used to correct for these vessels was also stated in the specification.

In the standard shown, the two auxiliary tubes (partly filled with mercury) at the extreme ends of the standard tube serve as current leads, while the platinum wires sealed into the lower ends of the other two auxiliary tubes form the potential leads.

Absolute measurements of resistance recently made show that the probable value of the international ohm is 1.00052 absolute ohms.