Mendenhall Order

USA Issued 5 April 1893 by Thomas C. Mendenhall, this order is usually seen as defining the US Customary units in terms of metric units instead of their own prototypes.

Mendenhall, long an advocate of the metric system, was at the time Superintendent of the Coast Survey, and thereby responsible for the modest national Office of Weights and Measures. His action, concurred with by the Secretary of the Treasury, effectively implemented a statute of 1866 that legalized the use of metric units. The values in the Order were

1 m	= 39.37 in,	giving 1 in	= 25.400 051~ mm
1 kg	= 2.204 622 34 lb,	giving 1 lb	= 453.592 43~ g

The former was precisely that of the 1866 statute; the latter differed from the statute slightly, because of the more recent appraisal of the reference standard. The exact expression of metric units in US units, as distinct from the exact expression of US (and British) in metric as applies now, pointed to the facilitating of conversion from metric to local customary units.

The 1866 statute, in authorizing the use of metric in contracts, etc., and specifying their values in normal US units, did not invalidate the basing of the national units on the prototype yard and pound. The Mendenhall Order, in contrast, is invariably interpreted as redefining those national units in metric, though argument has been made that, since it lacked legislated authority, it could not effect the consequent change, albeit minor, in those units. Indeed, it has been said that the order merely defined the metre and the kilogram for American consumption, changing them slightly in size! Whatever the legality, the reality is that it changed the basing of the US units. Mendenhall's definitions lasted until 1959 when slightly different international definitions became the law; they are:

1 in	= 25.4 mm,	giving 1 m	= 39.370 078 7~ in
1 lb	= 453.592 37 g,	giving 1 kg	= 2.204 622 62~ lb