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Building Science

The BTU-tiful History of the BTU

The British Thermal Unit gives us a handle on an abstract concept

Every year on New Year’s Eve, I head over to some friends’ house here in the Atlanta area. They have the best party around — Possum Drop — and I’ve been going for nearly a decade now. As midnight approaches, the newly-crowned Possum Queen leads the countdown. The possum is lowered slowly onto the fire and then erupts in an explosion flames and sparks.

Don’t worry. It’s not a real live possum. It’s a giant possum made of chicken wire, papier-mâché, fabric, and other scavenged materials… and it’s stuffed full of fireworks!

How much heat can you get from a Christmas tree?

Naturally, the backyard bonfire is fueled by Christmas trees, an abundant fuel source in the days leading up to Possum Drop. So you might wonder, how many Christmas trees does it take to burn the possum?

And once you ask that question, you’ve started thinking like a 19th century scientist… sort of. Take Thomas Tredgold and Nicolas Clément, for example. Both worked on a similar problem: relating a quantity of fuel to a result of burning that fuel. Both came up with similar definitions.

According to Dan Holohan, who is famous for helping to carry 18th century steam heating knowledge into the 21st century, Tredgold is credited with inventing the British Thermal Unit, or BTU. In his article The Origin of the British thermal Unit, Holohan quoted from Tredgold’s book: “I take as the measure of the effect of a fuel, the quantity, in pounds avoirdupois, which will raise the temperature of a cubic foot of water one degree of Fahrenheit’s scale.”

The definition has evolved a bit since then, though. Now we define it thus: One BTU is the amount of…

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  1. Charlie Sullivan | | #1

    Penny match vs. kitchen match
    The one-BTU match is actually the smaller wooden match size, sometimes called a "penny match," not a kitchen match.

    Most books and articles repeat this mistake uncritically, so when I first wondered about this, I weighed some matches to check. A kitchen match is clearly too big, even without counting any contribution from the head and the penny match was very close to one BTU. I'm afraid I did that measurement only to middle-school science standards, and I don't have error bars, but the conclusion was clear.

    But since this is a history article not a kitchen-science-experiment article, here's some history of the match example: The only source that I found that specifies clearly and correctly which match is an article in Advertising Agency Magazine, Vol. 48, no, 23, p. 26. The confusion may have arisen because the match example for BTU dates back to at least 1910, which was before large kitchen matches were introduced, sometime around 1930. An early mention of the match example appears in Will W. Wood, “How to use an air pump,” Brotherhood of Locomotive Firemen and Enginemen's Magazine, Volume 49, p. 497, 1910.

    Image from

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Charlie Sullivan
    Great research -- and an interesting footnote on the topic.

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