James Prescott Joule: A Life in Five Chapters

Chapter 1 · The Brewery

Dalton's pupil, and an income that asked no questions

1818 – 1840 · Salford · Manchester

James Prescott Joule was born at Salford near Manchester in December 1818, into a prosperous *brewing* family. He had a spinal condition and was frail as a child; he and his brother were educated at home.

From about 1834, when he was fifteen, he and his brother were taught for two or three years by *John Dalton*, then elderly and shortly to suffer a stroke. Dalton taught them arithmetic, geometry and chemistry, and — Joule always said — the habit of *measurement*.

He worked in the brewery. He was a working brewer for many years, and it gave him three things.

*Money* — an income, so he never needed a salary, a post or a patron.

*A workshop* — brewery cellars, apparatus, and craftsmen who could make things.

And, unexpectedly, *thermometry*. Brewing depends on temperature control: mashing, fermentation and conditioning all require holding a temperature within narrow limits, and a brewer who cannot measure temperature accurately produces bad beer. Joule grew up in an environment where precise thermometry was a *commercial necessity*.

He developed it far beyond that. He had thermometers made to his specification and calibrated them himself, and could reliably read to about a two-hundredth of a degree Fahrenheit — perhaps better. That is far beyond what most physicists of the period thought possible, and it is the whole basis of his work.

He never held a university post, never had a formal scientific position, and was, in the eyes of the scientific establishment, an *amateur*.

“The grand agents of nature are indestructible; wherever mechanical force is expended, an exact equivalent of heat is always obtained.”

— James Prescott Joule, On the Mechanical Equivalent of Heat (1843)

Why this matters

Joule's extraordinary thermometry came from a brewery, where holding a temperature within narrow limits is a commercial necessity rather than a scientific luxury.

You have the brewery, Dalton and the thermometers. What would you ask him?

Ask Joule

• “What did brewing teach you about measuring temperature?”
• “What did Dalton actually teach you?”
• “How accurately could you read a thermometer?”
• “What did having no post allow you to do?”
• “Did the brewery ever get in the way?”

Chapter 2 · The Square of the Current

Joule's law, 1841

1840 – 1843 · Manchester

Joule's first serious work came from a practical question. In 1838 he built an *electric motor* and wanted to know whether electric motors might replace steam engines in the brewery.

To answer that he had to measure how much *work* a motor produces for a given expenditure — and how much energy is *wasted* as heat.

In 1841 he published the result now called *Joule's law*: the heat produced in a conductor carrying a current is proportional to the *resistance* and to the *square of the current*.

The square is the important part and it is not obvious. Double the current and you get *four* times the heating.

This has enormous practical consequences, and they were not fully appreciated for decades. It is why a thin cable overheats while a thick one carrying the same current does not. It is why electrical transmission is done at *high voltage* — for a given power, higher voltage means lower current, and lower current means far less heating loss in the cables, because the loss goes as the square. It is why a kettle element, a toaster and a filament lamp work at all.

Joule's conclusion about the motor was negative: it was hopelessly uneconomic compared with steam, given the cost of zinc for batteries. He said so.

But the investigation had put him onto the real question. He had a machine converting one thing into another, and he began to wonder whether there is a *fixed rate of exchange* — whether a given amount of mechanical work always produces exactly the same amount of heat.

The prevailing view said no such question made sense, because heat was a *substance*: the *caloric*, a weightless fluid that is conserved, moved about, and cannot be created. On that account you cannot *make* heat out of work at all; you can only move existing heat from one place to another.

Why this matters

Heating goes as the square of the current, which is why electricity is transmitted at high voltage and why a thin cable overheats when a thick one does not.

You have the squared current and the question it raised. What is your question?

Ask Joule

• “Why does heating go as the square of the current?”
• “Why is electricity transmitted at high voltage?”
• “What made you build an electric motor in the first place?”
• “What was the caloric theory?”
• “Why does a thin wire glow and a thick one not?”

Chapter 3 · The Paddle Wheel

A fifth of a degree, and everything resting on it

1843 – 1847 · Manchester

The most famous experiment is the *paddle wheel*, and its logic is beautifully direct.

An insulated copper vessel contains a known mass of water. Inside it, a set of paddles is mounted on a shaft, with fixed vanes on the walls so the water cannot simply rotate as a body. The shaft is turned by a cord passing over pulleys, carrying *known weights*.

The weights fall through a *measured distance*. The work done is the weight times the distance — a quantity that can be calculated exactly.

The paddles churn the water. Friction between the water and the paddles converts that work into heat, and the water *warms*.

Measure the temperature rise, and you have the heat produced. Divide the work by the heat, and you have a *rate of exchange* between them.

The difficulty is the size of the effect. Falling weights of a few pounds, through a few feet, dissipating into a couple of pounds of water, produce a temperature rise of a *fraction of a degree*. In the 1845 version it is around a fifth of a degree Fahrenheit.

Everything depends on that being measured correctly, and on every source of heat loss being accounted for. Joule went to extraordinary lengths: an insulated vessel inside a wooden box, a screen between himself and the apparatus so his own body heat did not reach it, corrections for the heat lost to the surroundings during the run, corrections for the kinetic energy the weights retained on landing, corrections for friction in the pulleys.

Then he did the same thing in *other ways*. He forced water through fine tubes. He compressed and expanded air. He rubbed cast-iron discs together under mercury. He used the electrical heating from his own law.

All of them gave *the same number*.

That convergence is the argument. If a dozen entirely different ways of turning work into heat all give the same exchange rate, they are not separate phenomena.

Why this matters

A dozen entirely different ways of converting work into heat all gave the same exchange rate, and that convergence is what makes the result an argument rather than a measurement.

You have the falling weights and a fifth of a degree. What would you ask?

Ask Joule

• “Walk me through the paddle-wheel experiment.”
• “What had to be corrected for?”
• “Why do the experiment a dozen different ways?”
• “Why put a screen between yourself and the apparatus?”
• “How do you know the water is not just rotating?”

Chapter 4 · Cork, 1847

An unimpressed room, and a question from a young man

1843 – 1847 · Cork · Oxford · Manchester

Joule was dismissed for years, and it is worth being fair about why.

He was an *amateur*, a brewer from Manchester with no post and no degree. He was reading papers to the British Association rather than to the Royal Society, and the Royal Society rejected his early papers for its *Transactions*.

His claimed temperature changes seemed *too small to measure*. This was the substantial objection. Most physicists of the period could not read a thermometer to better than a tenth or a twentieth of a degree, and Joule was claiming to detect and rely on differences much finer than that. To a reasonable observer, his results looked like noise, and his confidence in them looked like inexperience.

And he was attacking the *caloric theory*, which was well established and had been productive.

He had also been anticipated, in a way. *Julius Robert von Mayer*, a German physician, had published the equivalence of heat and work in 1842 — arriving at it by a theoretical route, from the difference between the specific heats of a gas at constant pressure and constant volume. Mayer had almost no experimental support and was ignored more completely than Joule; a bitter priority dispute followed, and Mayer's mental health collapsed under it, with a suicide attempt and a period of confinement.

In June 1847, at the British Association meeting in *Oxford*, Joule was told to keep his presentation short and the room was largely unmoved.

Except for one person: a twenty-two-year-old professor from Glasgow named *William Thomson*, later Lord Kelvin, who asked a question.

Thomson was sceptical at first — he had been working within Carnot's theory, which assumed caloric — but he took Joule seriously, and he had the mathematical training Joule lacked. Their collaboration lasted decades and produced the *Joule–Thomson effect*: a gas cooling as it expands through a porous plug, which is the basis of refrigeration and of gas liquefaction.

Joule and his wife encountered Thomson again on their honeymoon in Switzerland, where Joule was attempting to measure the temperature difference between the top and bottom of a waterfall.

Why this matters

The objection to Joule was reasonable: he was claiming to detect temperature differences far finer than most physicists believed measurable, and he was right.

You have the unimpressed room and the one question. What is your question?

Ask Joule

• “Why did people think your measurements were too small to trust?”
• “What did Thomson ask you at Oxford?”
• “What did Mayer reach before you?”
• “Did you really measure a waterfall on your honeymoon?”
• “How do you keep going when nobody believes you?”

Chapter 5 · Nothing Is Lost

What the fixed exchange rate means

1847 – 1889 and after · Manchester · Sale

The full significance of a *fixed* exchange rate between work and heat is worth stating.

If a given amount of work *always* yields the same amount of heat, then work and heat are not two different things that happen to be related. They are two forms of *one quantity*, and that quantity is conserved: it can change form indefinitely but the total does not alter.

That is the *conservation of energy* — the first law of thermodynamics — and it is one of the most powerful principles in physics. Nothing is ever lost. Energy in a stretched spring, a raised weight, a hot body, a moving train, a chemical bond, a charged capacitor, sunlight: all are the same quantity in different forms, exchangeable at fixed rates.

It also rules things out permanently. A perpetual motion machine cannot work, ever, whatever its design, because it would require creating energy.

Joule supplied the *experimental foundation*. Mayer had reached it theoretically, and *Helmholtz* stated it as a general principle in 1847, generalising across mechanics, heat, electricity and physiology.

Joule's own contribution was the measurement — done repeatedly, by many routes, to a precision nobody else could match, until the number was undeniable.

Recognition came slowly and then fully. He was elected to the Royal Society in 1850 and received its *Copley Medal* in 1866, nearly twenty-five years after the work.

His later years were difficult. He was never a wealthy man by the standards of his class, the family *brewery was sold* in 1854, and he suffered financial losses. He was granted a *civil list pension* in 1878. His wife Amelia had died in 1854 after seven years of marriage, leaving him with young children.

His later work never approached the importance of what he did before he was thirty.

He died at Sale in October 1889, aged seventy. His gravestone carries the number 772.55 — his final value for the mechanical equivalent of heat, in foot-pounds per British thermal unit.

In 1889 the *joule* was already the unit of energy.

Why this matters

A fixed exchange rate between work and heat means they are two forms of one conserved quantity, which is why a perpetual motion machine cannot work whatever its design.

You have the number on the gravestone. What would you ask him?

Ask Joule

• “Why does a fixed exchange rate mean energy is conserved?”
• “Why can no perpetual motion machine ever work?”
• “How should the credit be shared with Mayer and Helmholtz?”
• “Why is a number carved on your gravestone?”
• “What happened after the brewery was sold?”

What Joule changed

The joule is the unit of energy in every laboratory in the world. Joule's demonstration that work and heat are interchangeable at a fixed rate is the experimental root of the conservation of energy, and Joule's law — heating proportional to resistance and to the square of the current — governs everything from a kettle element to why electricity is transmitted at high voltage.

A debate that continues

Julius Robert von Mayer reached the equivalence of heat and work independently and earlier by a theoretical route, and the priority dispute that followed contributed to a collapse in his mental health; how the credit should be divided between Mayer, Joule and Helmholtz is still discussed.

Keep exploring — ask Joule

• “Which of your experiments took the most patience?”
• “How do you convince a field that will not listen?”
• “What would you measure with a modern thermometer?”

Related lives

• Lord Kelvin — Who Found The Bottom Of The Scale
• Hermann von Helmholtz — Who Measured the Unmeasurable
• Joseph Black — Whose Thermometer Would Not Rise
• James Watt — Who Found Where the Steam Was Wasted

Related themes

Conservation of energy · Work and heat · Electrical heating