For other uses, see
Conservation of energy.
Joule's apparatus for measuring the mechanical equivalent of heat in which the "
work" of the falling weight is converted into the "
heat" of agitation in the water.
In the
history of science, the mechanical equivalent of heat was a
concept that had an important part in the development and acceptance of the
conservation of energy and the establishment of the science of
thermodynamics in the 19th century.
The concept stated that
motion and
heat are mutually interchangeable and that in every case, a given amount of
work would generate the same amount of heat.
History
The idea that heat and work are equivalent was proposed by
Julius Robert von Mayer (1842) and independently by
James Prescott Joule (1843). Similar work was carried out by
Ludwig A. Colding (1840-1843). Central to these developments, however, was Joule's famous 1843 paper, entitled "The Mechanical Equivalent of Heat", in which he published the value A for the amount of work W required to produce a unit of heat Q. Joule contended that
motion and
heat were mutually interchangeable and that, in every case, a given amount of
work would generate the same amount of heat.
Joule experimented on the amount of mechanical work needed to raise the
temperature of a
pound of
water by one degree
Fahrenheit and found a consistent value of 772.24
foot pound force (4.1550
J·
cal-1).
Though a standardised value of 4.1860 J·cal-1 was established in the early 20th century, in the 1920s, it was ultimately realised that the constant is simply the
specific heat of water, a quantity that varies with temperature between the values of 4.17 and 4.22 J·
g-1·
°C-1.
The change in unit was the result of the demise of the
calorie as a unit in physics and chemistry.
Priority
Both Mayer and Joule met with contemporary neglect and resistance owing to the eminence of the
caloric theory of heat. Colding's work was little known outside his native
Denmark.
Hermann Helmholtz probably first became aware of the principle through Joule's work, on which he based his definitive 1847 declaration of the
conservation of energy, but by 1862 he had come to credit both Joule and Mayer.
Also in 1847, Joule's presentation at the
British Association for the Advancement of Science in
Oxford was attended by the precocious and maverick
William Thomson, later to become Lord Kelvin. Thomson was intrigued but initially sceptical. Over the next two years, Thomson became increasingly convinced of Joule's theory, finally admitting his conviction in print in 1851, simultaneously crediting Mayer. Thus began a fruitful collaboration between the two men, mainly by correspondence, Joule conducting experiments, Thomson analysing the results and suggesting further experiments. The collaboration lasted from 1852 to 1856. Its published results did much to bring about general acceptance of Joule's work and the
kinetic theory in
England.
However, in 1848, Mayer had first had sight of Joule's papers and wrote to the
French Académie des Sciences to assert priority. His letter was published in the
Comptes Rendus and Joule was quick to react. Thomson's close relationship with Joule allowed him to become dragged into the controversy. The pair planned that Joule would admit Mayer's priority for the idea of the mechanical equivalent but to claim that experimental verification rested with Joule. Thomson's associates, co-workers and relatives such as
William John Macquorn Rankine,
James Thomson,
James Clerk Maxwell, and
Peter Guthrie Tait joined to champion Joule's cause.
On
May 18,
1850, Mayer attempted to commit
suicide, possibly in part owing to distress caused by the controversy.
However, in 1862,
John Tyndall, in one of his many excursions into
popular science and many public disputes with Thomson and his circle, gave a lecture at the
Royal Institution entitled On Force
[1] in which he credited Mayer with conceiving and measuring the mechanical equivalent of heat. Thomson and Tait were angered, and an undignified public exchange of correspondence took place in the pages of the
Philosophical Magazine, and the rather more popular Good Words. Tait even resorted to championing Colding's cause in an attempt to undermine Mayer.
Though Tyndall again pressed Mayer's cause in Heat: A Mode of Motion (1863) with the publication of Sir
Henry Enfield Roscoe's
Edinburgh Review article Thermo-Dynamics in January 1864, Joule's reputation was sealed while that of Mayer entered a period of obscurity.
Notes
^ The usage of terms such as work, force, energy, power, etc. in the 18th and 19th centuries by scientific workers does not necessarily reflect the standardised modern usage.
By electrical method Joule's constant,
Where,
V is the applied voltageI is the current passedt is the time (in seconds)m1 is the mass of calorimeterm2 is the mass of waterc1 is the specific heat capacity of calorimeter = 0.09 cal / g °Cc2 is the specific heat capacity of water = 1 cal / g °CT1 is the initial temperature of waterT2 is the final temperature of water
Further reading
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