Müller I. A history of thermodynamics. The doctrine of energy and entropy (1185104), страница 7
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Joule is bestknown for the discovery of the Joule heating of a current that runs througha wire. That heat is proportional to the square of the current. In the courseof those studies Joule conceived the idea that there might be a relationbetween the heating of the current and the mechanical power needed to turnthe generator.And indeed he established that relation and came up with a mechanicalvalue of heat which he expresses in the words28The amount of heat which is capable of raising [the temperature of] onepound of water by 1 degree on the Fahrenheit scale, is equal and may beconverted into a mechanical force which can lift 838 pounds to a verticalheight of 1 foot.29Joule’s memoir is full of tables with carefully recorded observations.
Hedescribes his experiments painstakingly, discusses possible sources ofexperimental error, and attempts to compensate for estimated losses. In thatsense his paper has set standards, although to this day thermal and, inparticular, caloric measurements are notoriously difficult, time-consumingand inaccurate to boot.And indeed, in later experiments – reported in a similarly exemplaryfashion in the article “On the temperature changes by expansion andcompression of air”30 – Joule obtains the values 820, 814, 795, and 760instead of the 838 pounds cited in his article of 1843.
And there were othervalues from other experiments so that in 1845 Joule proposed a mean valueof 817 pounds31 as the most likely one. In the letter to the editors of thePhilosophical Transactions he says:28J.P. Joule: “On the heating effects of magneto-electricity and on the mechanical value ofheat.” Philosophical Magazine, Series III, Vol. 23 (1843) p. 263ff, 347ff, 435ff.29The paper was read to the Section of Mathematical and Physical Sciences of theBritish Association, Convention in Cork on August 21st, 1843.30J.P.
Joule: Philosophical Magazine, Series III, Vol. 26 (1845), p. 369 ff.J.P. Joule: “On the existence of an equivalence relation between heat and the ordinaryforms of mechanical power’’. Letter to the editors of the Philosophical Magazine andJournal. Philosophical Magazine. Series III, Vol. 27 (1845), p. 205 ff.31222 EnergyJoule criticizes Carnot’s and Clapeyron’sanalysis of the steam engine, see Chap. 3He says: Since I hold the view thatonly the creator has the power to destruct,I agree with … Faraday, that any theory thatleads to the destruction of force is necessarilyfalse.32Fig. 2.4. James Prescott Joule. A pious version of the first lawEach one of your readers who is lucky enough to live in the romantic areasof Wales or Scotland could indubitably confirm my experiments, if hemeasured the temperature of a waterfall on top and at the bottom.
If myresults are correct, the fall must create 1° heat for a fall of 817 feet height;and the temperature of the Niagara will therefore be raised 1/5 of a degreeby the fall of 160 feet.Asimov33 writes that Joule in fact made that experiment at the waterfallhimself during his honeymoon when he and his wife visited a scenic waterfall.In 1850, after many more experiments, Joule came up with 772 which isa really good value, see below.34We have already seen that Joule knew Rumford’s work and, in fact, thathe tried to calculate the mechanical equivalent of heat from Rumford’sobservation.
This came out too high – 1034 foot-pounds – but it was closeenough to Joule’s spectrum of values that he could say that Rumford’sresult confirms our conclusions satisfactorily.35In the same postscript Joule says that he observed that water pressedthrough narrow tubes heats up, and that gave him yet another value, – 770foot-pounds. And he expresses his believe in the conservation of energy bysaying: I am convinced that the mighty forces of nature are indestructibleby virtue of the Creator’s: F I A T!To this day the conservation of energy is an assumption – welldocumented, to be sure, but still an assumption.
But like Mayer, Joule feelsthat he needs to prove the law. And since he cannot do that, he comes upwith strange formulations: We may a priori assume that a completedestruction of force is supposedly impossible, since it is obviously absurd,32J.P. Joule: “Temperature changes by expansion and compression of air.” PhilosophicalMagazine Series III, 26 (1845) p. 369 ff.33 I. Asimov: “Biographies….” loc.cit.34 J.P.
Joule (1850) loc.cit.35 Post Scriptum to Joule’s memoir of 1843. loc.cit.James Prescott Joule (1818–1889)23that the properties, with which God has endowed matter, could bedestructed.The attentive reader will have noticed that after Mayer had adjusted his heatequivalent to Joule’s better measurements – as mentioned before – he had425 mÎÞ1 heat 1 gram at Ïß height .1308ParisianfeetÐàLet us see how Mayer came up with those numbers: If 1308 feet is multiplied by5/9 to convert from °F to °C we obtain 727 feet, – considerably lower than any ofJoule’s numbers.
But then we must realize that an English foot is 30.5 cm whilethe Parisian one was 32.5 cm. Thus Joule’s value, as quoted by Mayer, was indeed772 English foot-pounds as stated before.Of course, foot-pounds are out nowadays. The older ones among the readersmay remember their university days, when they learned the mechanicalequivalent of heat in the form:1 calorie = 4.18 Joule.Yes, indeed, Joule is the modern unit of energy! It is equal to 1 kgm2/s2.Joule gets the honour, because he was most accurate for the time and hebacked up his figure with a large variety of careful measurements.36Actually, the calorie went also out as a unit when the SI units wereintroduced,37 and nowadays all energies are measured in Joule, be theymechanical, thermal, chemical, electric, magnetic, or nuclear.
This was agreat relief indeed for everybody concerned.A good case can be made that the first law of thermodynamics, the law ofconservation of energy, was the greatest discovery of the 19th century. Andhow was it received? We have already described how Mayer had to grovelin order to have his paper accepted for publication, and Joule fared nobetter. Asimov writes38His [Joule’s] original statement of his discovery was rejected by severallearned journals as well as by the Royal Society and he was forced topresent it as a public lecture in Manchester and then get his speechpublished in full by a reluctant Manchester newspaper editor for whomJoule’s brother worked as a music critic.36Of course, 418 m is not Mayer’s and Joule’s 425 m. The difference lies in the gravitational2acceleration 9.81 m/s , because Mayer’s grams and Joule’s pounds were weights, notmasses.
We have to correct for that.37 Système International d’Unites. It was introduced by international agreement in 1960.38 I. Asimov: “Biographies” loc.cit.The lecture was given on April, 28th 1847 in the St. Ann’s Church Reading-Room inManchester. It was published by the Manchester Courier on May 5th and May 12th.242 EnergyFortunately for him, the young, up-and-coming scientist WilliamThomson – later Lord Kelvin (1824–1907) – heard Joule speak andrecognized the quality of his research which he continued to advertisesuccessfully. In due course the two men became friends and collaborators.Joule was eventually able to measure 0.005°F reliably and the twoscientists – Joule and Kelvin – used such accurate measurements to showthat the temperature drops very slightly when a gas is allowed to expandinto vacuum.
This is now known as the Joule-Thomson effect – or theJoule-Kelvin effect – and it is due to the fact that the molecules of the gasupon expansion must run uphill in the potential energy landscape that isformed by the molecular attraction.39 This cooling effect proved to beimportant for the effort to reach lower and lower temperatures and bothJames Dewar (1842–1923) and Karl von Linde (1842–1934) made use of itin their efforts to liquefy gases and vapours, see below, Chap. 6.You cannot be an intelligent man and spend your lifetime measuringtemperature and heat without forming an idea what heat is. Rumford hadalready speculated that heat was motion and Joule says:40 I hold to thetheory which considers heat as a motion of the particles of matter and hequotes John Locke (1632–1704) who had said it all one and a half centuryearlier 41Heat is the very brisk agitation of the insensible parts of the object, whichproduces in us that sensation, from whence we denominate the object hot;so what in our sensation is heat, in the object is nothing but motion.Largely due to Kelvin’s propaganda, Joule’s work was widely recognizedand appreciated.
In 1866 he was awarded the Copley medal of the RoyalSociety, which Mayer also received, albeit 5 years after Joule. Toward theend of his life Joule’s brewery did not go well and he suffered someeconomic hardship. But he was saved by Queen Victoria who granted him apension.Hermann Ludwig Ferdinand (von) Helmholtz (1821–1894)42For centuries people had tried to construct a perpetuum mobile by arrangingmasses – and possibly springs – in the gravitational field, so that they would39This cooling effect is absent in a truely ideal gas, but quite noticeable in a vapour, i.e.
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