The Metric Act of 1866, enacted on , legally recognized the metric system of measurement in the US. It’s sometimes referred to as the Kasson Act, after Congressman John A. Kasson of Iowa, who chaired the House Committee on Coinage, Weights, and Measures. The history section below has more details on the reasons behind the law from John Kasson’s report to Congress.
Text of the law
The Act was codified as 15 USC 204 et seq., shown below. The following version shows the law as amended in 2007; refer to the history section for details.
US Code
Title 15
Commerce and Trade
Chapter 6
Weights and Measures and Standard Time
Subchapter I
Weights, Measures, and Standards Generally
Sec. 204. Metric system authorized
It shall be lawful throughout the United States of America to employ the weights and measures of the metric system; and no contract or dealing, or pleading in any court, shall be deemed invalid or liable to objection because the weights or measures expressed or referred to therein are weights or measures of the metric system.
Sec. 205. Metric system defined
The metric system of measurement shall be defined as the International System of Units as established in 1960, and subsequently maintained, by the General Conference of Weights and Measures, and as interpreted or modified for the United States by the Secretary of Commerce.
History
Original bill and amendments
The Metric Act of 1866 was originally introduced as H.R. 596 in the 39th Congress. The House passed it on ; the Senate passed it on ; and it was presented to the President and signed the next day.
The 1866 Act included a now-obsolete definition of the metric system and tables of units, which you can view in the original 1866 bill as passed by the House and sent to the Senate.
On , the 1866 law was amended by Pub. L. 110–69, the America COMPETES Act. It replaced the old definition of the metric system by the modern-day definition of SI. The relevant pages are available here (PDF, 5 pp, 77K), extracted from the 148-page law. The extract includes the cover page and Sec. 1, containing the short title of the act, then jumps to the page containing Sec. 3013(c)(1), which amends the Metric Act of 1866.
The extract also includes the rest of Sec. 3013, which repeals an obsolete law that defined electrical and photometric units and amends the law defining time zones, primarily to officially establish UTC (Coordinated Universal Time) as the basis for clock times.
Background
Following is some background on the reasons for the law, and the values of the conversion factors, from John Kasson’s report of the Committee on Coinage, Weights, and Measures (39th Congress, 1st Session, H.R. Report No. 62, ):
Chairman, House Committee
on Coinage, Weights, and Measures
The metric system is already used in some arts and trades in this country, and is especially adapted to the wants of others. Some of its measures are already manufactured at Bangor, in Maine, to meet an existing demand at home and abroad. The manufacturers of the well-known Fairbanks scales state: For many years we have had a large export demand for our scales with French weights, and the demand and sale is constantly increasing.
Its minute and exact divisions specially adapt it to the use of chemists, aphothecaries, the finer operations of the artisan, and to all scientific objects. It has always been and is now used in the United States coast survey. Yet in some of the States, owing to the phraseology of their laws, it would be a direct violation of them to use it in the business transactions of the community. It is therefore very important to legalize its use, and give to the people, or that portion of them desiring it, the opportunity for its legal employment, while the knowledge of its characteristics will be thus diffused among men. Chambers of commerce, boards of trade, manufacturing associations, and other voluntary societies, and individuals, will be induced to consider and in their discretion to adopt its use. The interests of trade among a people so quick as ours to receive and adopt a useful novelty, will soon acquaint practical men with its convenience. When this is attained—a period, it is hoped, not distant—a further act of Congress can fix a date for its exclusive adoption as a legal system. At an earlier period it may be safely introduced into all public offices, and for government service.
In the schedule of equivalents provided in the bill, extreme scientific accuracy is not expressed. The reasons follow. The exact length of the meter in inches and the weight of the kilogram in grains can of necessity be determined only approximately. The most careful determinations of these quantities now possible are liable to minute corrections hereafter, as more numerous observations are made and better instruments are used. Instead, therefore, of aiming at an accuracy greater, perhaps, than is attainable, it is more expedient to consult the convenience of the people by using the simplest numbers possible in the schedule, and yet such as shall be in fact more nearly exact than can ever be demanded in the ordinary business of life. These numbers are to be used in schools, and in practical life millions of times as multipliers and divisors, and every unnecessary additional figure is justly objectionable.
In a popular sense of the word, however, the numbers in the schedule may be said to be exact. The length of the meter, for example, is given as 39.37 inches. The mean of the best English and the best American determinations differs from this only by about the amount by which the standard bar changes its length by a change of one degree of temperature. Such accuracy is certainly sufficient for legal purposes and for popular use.
You can also view images of some relevant pages from the 1866 Journal of the House of Representatives and Journal of the Senate on its introduction and passage.
You can view a handwritten original of the bill at the National Archives and Records Administration (NARA): page 1 and page 2.
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