James Prescott Joule

1818 - 1889
by David F. Coppedge

If any principle in science deserves to be called a “law,” what would it be? Undoubtedly, the law of conservation of matter and energy: neither of these fundamental entities can be created or destroyed. Also known as the first law of thermodynamics, this law has no known exceptions anywhere in the universe. Whoever discovered this law must have been a scientist of the highest rank, a PhD, director of a reputable university research department, respected the world over, and interred in Westminster Abbey, right? Actually, he was none of the above. For him, science was just a hobby. He had trouble getting his ideas published. Professional scientists looked down on him, and were it not for the help of a friend, his work might have been lost in obscurity. Yet his experimental procedures and measurements were of the highest caliber, and the principles he deduced from them are of fundamental importance. They helped shape our modern world, and every housekeeper is a beneficiary of the discoveries he made. Units and laws of physics were named after this somewhat reserved, unassuming, serious-minded citizen scientist by the name of James Prescott Joule.

Second son of a wealthy brewer in England, James Joule was home-schooled till age 15. He was not a spoiled rich kid, even though he could spend a workman’s annual income on a painting if he wanted it (and once did). James loved playing outdoors with older brother Benjamin and younger brother John. Together they engaged in the typical boyish amusements like playing guns, rowing on the lake, climbing hills and throwing snowballs. Their play included observational skills like measuring the depth of a lake, estimating the distance to a lightning bolt by timing the thunder, and using electricity to see if a lame horse’s muscle would jump. Once as a young man he stuffed a pistol with three times the normal charge trying to get a better echo across the water; J. G. Crowther describes the scene: “His brother was startled by a tremendous report and when he turned round he found that James’ pistol had jumped out of his hand into the lake. At another time he shot off his own eyebrows.” Boys will be boys, but at least theirs were not idle minds. The brothers had a variety of interests; Benjamin was an enthusiastic musician, and James developed skill in painting and photography; he even collected art. At 16, James had been sent to Cambridge and was tutored for a time by John Dalton, the elderly Quaker scientist considered to be the father of modern atomic theory. Spurred by in an interest in science, and having the family wealth at his disposal, James took a keen interest in devising experiments to measure things: heat, energy, motion, electrical currents, magnetism and gas pressures. Like Faraday, he expected to find simple laws that governed diverse natural phenomena, a motivation that derived from strong theological beliefs.

Joule’s device for measuring the mechanical equivalent of heat

Throughout his twenties, working at his father’s brewery, young James Joule was actively demonstrating through a series of clever experiments that different forms of energy were related. For instance, he measured the temperature of water being forced through narrow holes in a piston. He measured electrical current and heat output from an electromagnet that spun as he turned a crank. And he measured the temperature of water and sperm whale oil as paddles turned, powered by falling weights, proving that heat output was equivalent to the mechanical energy input (see illustration). The precision of his measurements was remarkable, sometimes measuring temperatures accurate to a 30th of a degree. His numerous creative experiments convinced him that all forms of energy were equivalent, to the point where he said in 1843 at age 24, “I shall lose no time in repeating and extending these experiments, being satisfied that the grand agents of nature are by the Creator’s fiat, indestructible; and that wherever mechanical force is expended, an exact equivalent of heat is always obtained.” When his paper was eventually published in the Philosophical Magazine in 1845, he expressed again his belief in a Creator: “Believing that the power to destroy belongs to the Creator alone I affirm … that any theory which, when carried out, demands the annihilation of force, is necessarily erroneous.”

Joule had discovered the mechanical equivalent of heat, but the scientific community was not ready to accept it. Though the caloric theory of heat had been discredited by the late 1700s, heat was still considered a property of a body, not a form of energy released by work in converting one form of energy to another. In 1843, he journeyed to Cork and read a paper describing his experiments to the Chemical Section of the British Association, but he says, “the paper did not excite much attention,” except for two who “were interested.” Polite disdain, perhaps, but in retrospect, J. G. Crowther, author of British scientists of the 19th Century, thinks more highly of it:

Reynolds [biographer of Joule, Memoirs, 1892] considers the experiments described in this paper were technically the most difficult that had ever been accomplished by a physicist. They are certainly unsurpassed in the history of science.

The combination of superb experimental skill with clear thought and philosophical depth makes this paper the finest expression of Joule’s genius. He was twenty-four years of age, and had been engaged in research for five years. Though he was friendly with Dalton, Scoresby, Davies and others, he had worked in extraordinary intellectual independence. His chief supports were his own genius and his father, who, to his memorable credit, liberally financed his extensive experiments.

Crowther finds it remarkable that young Joule was so meticulous in measuring things, because “young scientists are nearly always impatient of measurement. Joule had the middle-aged passion of measurement from his earliest youth.” His notes are equally meticulous, orderly and filled with profound insight into the implications of the measurements. He wrote other papers during his twenties, one comparing the capabilities of electromagnets, steam and horses as sources of motive power. At age 28, his genius matured with a lecture that contained a philosophical statement of the law of conservation of energy.

Memorial Hall, Manchester, built 1863-1866, would have been familiar to Joule in his mature years as a new landmark in his home town.

In a groundbreaking lecture, Joule stated that bodies carry with them a “living force” of inertia, and it cannot be destroyed “though that was the common opinion of philosophers.” Friction does not destroy it, or the earth would have come to a standstill long ago, he said. Rather than being destroyed, it was transformed into another thing when it disappears: that thing is heat. He had proved experimentally that heat and work are equivalent and can be converted one to the other. Joule demonstrated his grasp of this laboratory principle by extending it to the motion of the earth, the burning of meteors, the motion of the trade winds and the heat generated by motion of our limbs, as when a man ascends a mountain. Joule showed how his dynamical theory of heat explains melting, latent heat, evaporation, and much more. “We may conceive, then, that the communication of heat to a body consists, in fact, in the communication of impetus, or living force, to its particles.”

Crowther is unreserved about the import of this lecture: “He had discovered the law [of conservation of energy] as the outcome of a long series of completely conclusive experiments. He had conceived it clearly and powerfully, and applied it with much imagination.” So where was this epochal lecture On Matter, Living Force, and Heat delivered? At the Royal Society or the British Association? No: at St. Anne’s Church in Manchester, and Joule had trouble getting it published. The Manchester Guardian only wanted to print excerpts of their choosing. James needed his brother’s persuasion to convince the Manchester Courier to print it, which they did in two parts in May, 1847. Since it was published in a newspaper instead of the scientific journals, it went virtually unnoticed for 37 years.

The month following the publication of this lecture, Joule had an opportunity to address the British Association about his experiments on the mechanical equivalent of heat. “As Joule’s previous papers had raised little interest, the chairman of his section requested him to confine himself to a short verbal description of his experiments,” writes Crowther. A contemporary described James Joule as “under the medium height; that he was somewhat stout and rounded in figure; that his dress, though neat, was commonplace in the extreme, and that his attitude and movements were possessed of no natural grace, while his manner was somewhat nervous, and he possessed no great facility in speech.” So the short and stout and nervous Joule endeavored to make it quick, and commented that “the communication would have passed without comment if a young man had not risen in the section, and by his intelligent observations created a lively interest in the new theory.” That man was William Thomson – the future Lord Kelvin (see Kelvin biography in this series).

Though Thomson was seven years his junior, he had the connections to bring James Joule into scientific circles. Their collaboration developed into a lifelong friendship. About a week after this meeting, Joule married Amelia Grimes, daughter of a city official. Thomson was surprised another week later to run into Joule near Mont Blanc, and find him with a lady, not knowing he was getting married, and here he and his bride were on their honeymoon. He was probably more surprised to see him with a long thermometer in his hand. Joule explained that he wanted to measure the temperature elevation in waterfalls, so Thomson offered to join the fun and help him a few days later with this project, another demonstration of the mechanical equivalent of heat. Unfortunately, they found their chosen cascade too broken up into spray to get good data. Was Amelia put off by this intrusion into their romantic vacation? Not at all; Crowther writes, “His young wife, as long as she lived, took complete interest in his scientific work.”

Amelia died in 1854 after just seven years of marriage, having given birth to a son and a daughter. James took the children with him to live with his father, but soon experienced other losses; his father died in 1858, and in the same year, James was a victim of a train derailment when a stray cow got onto the tracks. The carriage in which he had been reading a mathematics book overturned, and he had to crawl out for his life – only to find the engine men nonchalantly eating their dinner, apparently unconcerned that three people had died in the accident. This made him somewhat phobic about riding trains from then on. In 1864, his younger brother died. In spite of these traumas, his collaboration with Thomson grew productive and soon yielded more discoveries of fundamental importance.

Joule respected Thomson’s mathematical abilities, and tended to play second-fiddle to the Glasgow professor, acting as his chief laboratory assistant, even though he possessed enough of his own genius to be his peer. Perhaps physical or psychological ill-health from recent trials affected his self confidence. Nevertheless, they made a great team. Do you like having a refrigerator in the kitchen? Here’s the story; it comes right out of this historic collaboration, and it took Joule seven years of difficult – and dangerous – experiments. The new theory of thermodynamics was driving physics at the time; Joule and Thomson were at the crest of the wave. Joule measured air as it compressed and expanded, and found that it departed just slightly from Boyle’s Law for an ideal gas. From this, they deduced that air should cool slightly if allowed to expand through a small hole without performing any work. Thomson suggested Joule prove this with experiments.

Small-scale tests showed promise, but Joule decided he needed a bigger apparatus powered by a 3-horsepower steam engine to get more reliable measurements. The Royal Society provided the funds, and the machine was built. For the first year, he was able to operate it at the family brewery. But in 1854 the brewery was sold, so he had to move the contraption to his house in Manchester, with some of it sticking out in the open air because his lab was not large enough to contain it all. His older brother described the scene: for several months James “could not find time to take his meals properly–just ran in and out again. The experiments were so delicate that many were carried out in the night, because a cab or cart passing along the road disturbed them, though the laboratory was at the back of the stables” (Crowther, p. 193).

Joule had to transport the contraption again in 1861 when he moved to a new house, but then a neighbor complained about the commotion so much he got the authorities to put a stop to it. “James was deeply upset by this action,” Crowther says. Nevertheless, after seven years working on the thermal properties of gases with Thomson, they published their crowning achievement, an explanation of the cooling as being due to the absorption of heat in the performance of work separating molecules that have a slight mutual attraction. This is the Joule-Thomson effect, the basis of liquid air production and the refrigeration industry. The rest is history; the refrigerator today is one of the most-used electrical appliances in the home, allowing families to cool and freeze food, preserving it for long periods without the chore of calling the ice man every few days to deliver big blocks of ice that had been stashed during the winter. But that’s not all; we haven’t yet mentioned Joule’s Law, an important equation known by every electrician. It relates electrical power to resistance and current, and is the basis of the space heater and toaster and electric range; current forced through a strong resistor like nichrome wire generates power proportional to the resistance and to the square of the current. All that power is output as heat. When you watch that wire turn red, you are watching Joule’s Law at work.

At age 57 Joule’s money had run out, and he became poor while working on a more detailed verification of the mechanical equivalent of heat. Fortunately, the Royal Society funded the work, and the queen provided him a pension to live on. He published this, his last paper, in 1878, a year after the impressive Manchester Town Hall was completed (which would later house a statue of Joule and of John Dalton, who had tutored Joule when a young man). James lived out his final 11 years in relative privacy till succumbing to a long illness at age 71. Clerk Maxwell said of him, “There are only a very few men who have stood in a similar position and who have been urged by the love of some truth, which they were confident was to be found though its form was as yet undefined, to devote themselves to minute observations and patient manual and mental toil in order to bring their thoughts into exact accordance with things as they are.”

Original artwork by J. Beverly Greene commissioned for this biography. All rights reserved.

The Royal Society, who had years earlier paid little attention to this non-professional hobbyist, venerated Joule in his old age. He was described as “kindly, noble, and extremely chivalrous, but hated quackery, especially from persons of standing.” As one who had been disparaged himself, “he encouraged the efforts of workers as yet unknown and resented disparagement of their work, ‘as though his own early experience had left him with a fellow-feeling with those who were struggling’ to secure recognition of their results” (Crowther, p. 144). J. G. Crowther thinks Joule’s life resembles that of Leonardo da Vinci, in that they both pursued perfection, and “continued the refinement of technique, subtle thoughts following the solitary contemplation of the results of their accessions of manual skill.” Joule needs no stone monument in a cemetery; your home is filled with them, and from now on, you will no doubt remember this unique character with the magnificent full beard when you use your refrigerator, space heater, hair dryer, toaster, iron, or any of the other modern appliances that soon sprang from his discoveries in fundamental physics. Most important, Joule’s proof of the law of conservation of energy is one of the supreme achievements of modern science. Of this most basic and universal of all scientific laws, Henry Morris writes, “It is surely appropriate that the privilege of making such a vital discovery was given by God to a man of sincere Christian faith.” (Scientists of Faith, p. 53.)

To understand fully the motivation that makes a man like James Joule work so long and hard, often alone, we need only hear his own thoughts. Here are excerpts found on loose sheets of paper after his death of what Crowther believes was to be an address to the British Association in 1873. Joule had been elected President, but due to ill health had to resign, so the address was never delivered. It’s about time for the world to hear the wisdom of these words, because rarely has such a clear statement been given on why science should be the enthusiastic pursuit of the devout Christian. In the notes, Joule talks about many things; the value of science education for the youth, his opposition to science being applied to warfare or politics, the value of mathematical rigor, and the need for precision and planning in experimentation. He describes the ideal moral character of the scientist: one must be humble, diligent, energetic, prudent and zealous, pursuing science due to “a love of wisdom which unfolds, a love of truth for its own sake independently with regard to the advantages of whatever kind are expected to derived from it.” Science and knowledge elevate us above the beasts that perish, and enriches our lives with “varied and fresh enjoyments.”

Among these varied but uplifting thoughts (see below for full text), we end with two quotes that so well express the theme of this book, that good science – the best science – is the fruit of devout love of God as Creator. To Joule, the study of nature and her laws is “essentially a holy undertaking,” second only to worship as the rightful response to the Maker of all things. Hear the words of James Prescott Joule:

After the knowledge of, and obedience to, the will of God, the next aim must be to know something of His attributes of wisdom, power and goodness as evidenced by His handiwork. It is evident that an acquaintance with natural laws means no less than an acquaintance with the mind of God therein expressed.

In science labs and hardware stores around the world today, the most fundamental property of the universe – energy – is measured in joules.


James Prescott Joule

1818 – 1889 


The following excerpts are taken from J.G. Crowther, British Scientists of the 19th Century, (London, K. Paul, Trench, Trubner & co., ltd. 1935), pp. 138-140. Bold quotes are by James Joule, while unquoted text is that of Crowther. Reproduced verbatim, except the bold type added for clarity.

In addition to the note-books there are some loose sheets of paper whose contents are described by Dr. H. Lowe as “rough notes on the scope and methods of physical science” and “the aims of science and the true spirit of research.”

The writer believes these notes may be the draft of an address Joule was to deliver in 1873, as President of the British Association meeting at Bradford. Owing to ill health Joule had to resign the Presidency, so he did not deliver the address.

Joule writes that natural philosophy is second only to religion. “After the knowledge of, and obedience to, the will of God, the next aim must be to know something of His attributes of wisdom, power and goodness as evidenced by His handiwork.”

Sketch adapted from statue of Joule in the Manchester Town Hall, by J. Beverly Greene.

“The study of nature and her laws“ is “essentially a holy undertaking” and is of “great importance and absolute necessity in the education of youth.”

This pursuit of science is due to “A love of wisdom which unfolds, a love of truth for its own sake independently with regard to the advantages of whatever kind which are expected to be derived from it.” It is inspired by “a certain inquisitiveness of mind to know that which is already unknown, a principle which is one of the most important that belongs to our nature and is in fact the principal cause that reconciles us to life which would be miserable indeed if presented merely the recurrence of the same objects or gave no hope of varied and fresh enjoyments.”

The scientist must be humble, diligent, energetic, ardent and zealous. He should “receive a healthy stimulus from a well-regulated love of approbation, and the hope of success.”

The first object of natural science is to elevate humanity above the scale of creatures, and the second is to promote well-being. “The first object is therefore at least as much more important than the second as the intellect is more noble than the body.”

“It is evident that an acquaintance with natural laws means no less than an acquaintance with the mind of God therein expressed.”

Concerning applied science, “Nothing is more expensive than the endeavour to stumble blindly on an improvement or an invention.”

Speaking of the lack of scientific knowledge in the designers of the early submarine cables, he remarks that such knowledge “ought to be possessed by a gentleman in whose position one would certainly require; what is however often found, as accurate knowledge of physical science as is commonly possessed by a joiner or a mechanic.”

He pleads for education in science, and the communication of science to the public. Man without science is half-educated. “In the first place we may remark that the trite proverb that a little knowledge is a dangerous thing, absurd in other cases, is peculiarly so in this. This doctrine of fools would necessarily discountenance any education whatsoever because in passing from ignorance to the highest state of intellectual acquirement a man must be at one time induced with the dreaded little knowledge. The truth is that a little knowledge is a little good and much knowledge is a great good, while ignorance is an unmitigated evil allying us with the beasts that perish.”

Mathematics is of great importance to the scientist, but even advanced workers should frequently return to the simple truths. The analyst must continually recur to the descriptive facts from which he started.

Joule includes one passage with a tone remarkably familiar to our own days of 1935 [i.e., when Crowther was writing]. “Such then are the legitimate objects of science. It is deeply to be regretted that another and most unworthy object has been introduced and has gradually and alarmingly increased in prominence. This is the improvement of the art of war and the implements of mutual destruction. I know there are those who think that these improvements will tend to put an end to war by making it more destructive. I cannot think that such an opinion is based on common sense. I believe war will not only be more destructive, but be carried on with greater ferocity. Individual campaigns will doubtless be short as well as decisive, but this will necessarily cause the rapid rise and fall of states and unsettling of boundaries and constitutions which must eventually deteriorate civilization itself and render peace impossible. And thus by applying itself to an improper object science may eventually fall by its own hand. In reference to this subject we must also deplore the prostitution of science for the aggrandisement of individuals and nations, the result being that the weaker are destroyed and the stronger race is established on its rules. In making the above remarks I allude to war generally, and intend no disparagement of the effects being made to secure the integrity and liberties of Great Britain. These have been forced upon us and it is a matter of congratulation that we are not responsible for the present military attitude of Europe.”

Crowther concludes this section by describing Joule’s words as “expressions of his mature and experienced mind, which are so acute and in their political application so characteristically British.”

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