In Britain, late 19ᵗʰ century, a unit of electric energy = 1 kilowatt-hour. Abbreviation, “B. T. u.”, or, earlier, ”B.O.T. unit“.
“The legal or Board of Trade unit of energy for commercial electric lighting. = 1000 ‘watt-hours’....A ‘20-unit’ dynamo is one that gives a current of 20,000 volt-amperes, and therefore gives 20 units per hour.”¹
In 1892 the Board of Trade proposed naming this unit the kelvin, but Lord Kelvin demurred, saying he might go into the meter manufacturing business himself and it would be unfair to competitors to have every manufacturer's meters marked with the name of his company.²
1. Latimer Clark.
A Dictionary of Metric and Other Useful Measures.
London: E & F.N. Spon, 1891.
2. Letter from Lord Kelvin to Sir Courtney Boyle, 6 May 1892.
The Board of Trade unit for electric lighting, to which some have given the atrocious name of “Bot,” represents the energy expended by 1000 amperes under the pressure of one volt, during one hour; that is, it represents 1000 volt ampere hours.
John T. Sprague.
Electricity: Its Theory, Sources, and Applications. 3rd edition, revised and extended.
London: E & F. N. Spon, 1892.
A unit of electrical resistance, sometimes called the B. T. u. ohm, the standards for which were kept by the Board of Trade.
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Last revised: 3 July 2007.