Just What On Earth Is A Therm? [Hackaday]

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With the news here in Europe full of the effect of the war in Ukraine on gas supplies and consequently, prices, there it was on the radio news: a unit of measurement so uniquely British that nobody uses it in the real world and nobody even has a clue what it really means. We’re speaking of the Therm, one of those words from our grandparents’ era of coal gas powered Belling cookers and Geyser water heaters hanging over the bath, which has somehow hung on in the popular imagination as a mysterious unit of domestic gas referred to only in the mass market news media. What on earth is a therm, and why are we still hearing it on the news in the UK?

You can’t Buy A Therm

Asking the internet what a therm is reveals the answer, it’s 100,000 BTU. What’s a BTU? A British Thermal Unit, another anachronistic measurement five decades after the UK went metric, it’s the amount of energy required to raise a pound of water by a degree Fahrenheit. Which in turn is about 1,054 joules, in today’s measurements. So a therm is thus a unit of energy, can we take a look at our gas meters and see how many of them we’ve used this winter? Not so fast, because gas isn’t sold by the therm. Older gas meters had cubic feet on them, and we’re guessing that now they’re calibrated in cubic meters. We can’t even buy a therm of gas, so why on earth are the British media still using it?

To answer that question it’s fair to say that there are two reasons for the warm and cosy grip of the therm on the national discourse here. The first is that surprisingly, wholesale gas is traded in therms, so while we consumers buy it by volume, our utility companies buy it by energy. At the time of writing a therm of wholesale natural gas costs about £260 (about $342) to them, but given the geopolitical situation of the moment it’s anybody’s guess where it’ll be tomorrow. So when the price of gas is quoted in therms on the news it’s because somewhere a utility company is still buying the things — who knew!

Mr. Therm, He’s Hot Stuff!

But the other reason for the news media’s fondness for the term is cultural. We’d never have heard of the therm and it would have languished as an obscure engineer’s unit of energy derived from gas, were it not for the gas industry’s mascot. Mr. Therm was a cartoon character used to promote the gas industry and gas products from the 1930s until the 1970s, and his ubiquity gave the word a hold over the popular imagination that must still be there for the older generations. Perhaps the papers and newsrooms still fondly cling to Mr. Therm, or more likely, they recognise that it’s mostly older people who still buy printed newspapers.

The therm then, a unit of measurement nobody uses and nobody knows what it is, but one which lingers on in a corner of the gas industry and in fond memories of a world long past. Perhaps it’s best to think of it as a customary measurement in an old country, like a pint of beer — the real unit is the familiar sized glass. We’ll keep our 29.3 kWh, thank you very much.

Header: George Shuklin, Public domain.