| Below is an excerpt from Professor Osborne Reynolds, On the
Mechanical Equivalent of Heat, Philosophical Transactions
of the Royal Society of London, 66:601-733 (1897). The determination by Joule, in 1849, of the
expenditure of mechanical effect (772.69 lbs. falling 1 foot)
necessary to raise the temperature of 1 lb. of water, weighed in
vacuo, 1° Fahr. Between the temperatures of 50° and 60° Fahr. (at
Manchester), together with the second, in 1878, 772.55 ft.-lbs.,
at the latitude of Greenwich, established once for all the
existence of a physically constant ratio between the work expended
in producing heat and the heat produced; while the extreme
simplicity of his methods, his marvelous skill as an experimenter,
and the complete system of checks he adopted, have led to the
universal acceptance of the numbers he obtained as being within
the limits he himself assigned (1 foot), of the true ratio of work
expended in his experiments in producing heat and the heat
produced as measured on the scale of the thermometer on which he
spent so much time and care. The acceptance of J = 772, as the mechanical
equivalent of heat, amounts to the acceptance of the scale between
50 and 60 on Joule's thermometer b as the standard of
temperature over this range. Joule's thermometers are now in the custody
of the Manchester Literary and Philosophical Society (having been
confided to its care by Mr A. Joule); so that this material
standard is available. But the standard of temperature actually
established by Joule is universally available wherever the British
standard of length is available, together with pure water and the
necessary means and skill of expending a definite quantity of work
in raising the temperature of water between 50° and 60° Fahr.,
since in this way the scale on any thermometer may be compared
with that on Joule's. The difficulty of access to Joule's thermometer, and
the inherent difficulty of making an accurate determination of the
equivalent, have limited the number of such comparisons. The most serious attempts have been made with the very
desirable object of determining the mechanical equivalent of a
thermal unit, measured on the scale of pressures of gas at
constant volumes, first recognised by Joule as the nearest
approximation to absolute temperature. The results of these comparisons have been various,
all having apparently shown that Joule's standard degree
of temperature is less than the one-hundred-and-eightieth part
between freezing and boiling points on the scale of pressure of
gas at constant volume, the differences being from 0.1 to 1.0
percent. Joule himself contemplated comparing his thermometer
with the scale of air pressures, but did not do so. So that only
indirect comparisons have been possible. Hirn, who was the first to follow Joule, in one of his
researches introduced a method of measuring the work done which
afforded much greater facility for applying the work done to the
water than the falling weights used by Joule in his first
determination, and this was adopted by Joule in his second
determination. But notwithstanding the greater facilities enjoyed
by subsequent observers, owing to the progress of physical
appliances, the inherent difficulties remained. The losses from
radiation and conduction could only be minimised by restricting
the range of temperature, and this insured thermometric
difficulties, particularly with the air thermometer, which, it
seems, does not admit of very close reading. This, together with
certain criticisms, of which some of the methods employed admit,
appear to have left it still an open question what exact rise in
the temperature in the scale of air pressures corresponds to the
772 ft.-lbs. ...The institution of an air thermometer was carefully
considered and rejected. But it occurred to me that it might be
possible to avoid the introduction of scales of the thermometers,
just as before, and yet obtain the result. If it could be so
arranged that the water should enter the brake at the temperature
of melting ice and leave it at the temperature of water boiling
under the standard pressure, all that would be required of the
thermometers would be the identification of these temperatures.
At first the difficulties appeared to be very formidable. But on
trying, by gradually restricting the supply of water to the brake
when it was absorbing some 60 H.-P., and finding that it ran quite
steadily with its automatic adjustment till the temperature of the
effluent water was within 3° or 4° of 212° Fahr., I further
considered the matter and formed preliminary designs for what
seemed the most essential appliances to meet the altered
circumstances. ...The entire system of working was designed to secure
the most perfect elimination of radiation possible. Thus, it was
arranged in the first place that the trials be made in pairs, one
heavy trial and one light trial, made under circumstances as
nearly similar as possible, except in respect of load and water.
The loads in the first instance being 1200 and 600 foot-pounds,
and the quantities of water such that the final temperature should
be as nearly as possible 212° Fahr., and, after the preliminary
trials, 300 revolutions per minutes was adopted as the speed for
all the trials, 60 minutes as the time of running. The inlet and
outlet thermometers to be read after the first minute, and every
two minutes; also the temperature of the laboratory as shown by a
thermometer in a carefully-chosen place. This temperature to be
maintained as nearly constant as possible. The setting of the
regulators during each trial to be recorded; also the pressure of
the artificial atmosphere, and that in the supply pipe after
passing the coil; and, subsequently, the reading of the
thermometers in the stuffing-box and bearings taken every five
minutes, and the speed gauge every two minutes. The observations
and incidents being recorded by the rules in surveying, in ink, in
a book, and distinct from any reductions. The initial and final
reading on the scales and counter being included, as were also the
initial and final readings of the inlet and outlet thermometers
and speed gauge for the purpose of determining the terminal
differences of the heat in the brakes. As it was impossible to make trials simultaneously,
and so secure similar conditions in the laboratory, it was at
first arranged that the trials should be made in groups, including
four pairs of trials. |
