James Joule · Work and heat
Why does stirring make water warmer?
That time, that place
Into the early nineteenth century heat was thought to be a substance: caloric, an invisible fluid flowing from hot to cold.
But boring a cannon barrel was strange. With a blunt tool you could go on and on and the heat never stopped. If caloric were a substance it should eventually run out.
Why this question
Joule was a Manchester brewer's son. With no university post, he experimented while running the brewery.
His apparatus: paddles inside a vessel of water, turned by a falling weight. The height of the fall gives the work put in; the temperature of the water gives the heat that came out.
The rise was tiny. He built a thermometer that could read to a two-hundredth of a degree.
What was found
The same work always produced the same heat. Stirring water, stirring mercury, passing a current, even compressing air — the ratio held.
Heat was not a substance but a form of energy. Work had been converted, not drained away.
The first law of thermodynamics stands here: energy changes form and is neither created nor destroyed. The story goes that he measured the water temperature above and below a waterfall on his honeymoon.
Heat was explained as caloric, an invisible substance
Work delivered by different means always yielded the same heat
Conservation of energy stood, and engines became calculable