The method corresponding to the expression C^2RT was adopted by Joule
and by most of the early experimentalists. The defects of the earlier
work from an electrical point of view lay chiefly in the difficulty of
measuring the current with sufficient accuracy owing to the imperfect
development of the science of electrical measurement. These difficulties
have been removed by the great advances since 1880, and in particular by
the introduction of accurate standard cells for measurements of
electrical pressure.
S 12. _Griffiths_.--The method adopted by E.H. Griffiths (_Phil.
Trans._, 1893, p. 361), whose work threw a great deal of light on the
failure of previous observers to secure consistent results,
corresponded to the last expression E^2T/R, and consisted in
regulating the current by a special rheostat, so as to keep the
potential difference E on the terminals of the resistance R balanced
against a given number of standard Clark cells of the Board of Trade
pattern. The resistance R could be deduced from a knowledge of the
temperature of the calorimeter and the coefficient of the wire. But in
order to obtain trustworthy results by this method he found it
necessary to employ very rapid stirring (2000 revolutions per minute),
and to insulate the wire very carefully from the liquid to prevent
leakage of the current. He also made a special experiment to find how
much the temperature of the wire exceeded that of the liquid under the
conditions of the experiment. This correction had been neglected by
previous observers employing similar methods. The resistance R was
about 9 ohms, and the potential difference E was varied from three to
six Clark cells, giving a rate of heat-supply about 2 to 6 watts. The
water equivalent of the calorimeter was about 85 grammes, and was
determined by varying the quantity of water from 140 to 260 or 280
grammes, so that the final results depended on a difference in the
weight of water of 120 to 140 grammes. The range of temperature in
each experiment was 14 deg. to 26 deg. C. The rate of rise was
observed with a mercury thermometer standardized by comparison with a
platinum thermometer under the conditions of the experiment. The time
of passing each division was recorded on an electric chronograph. The
duration of an experiment varied from about 30 to 70 minutes. Special
observations were made to determine the corrections for the heat
supplied by stirring, and that lost by radiation, each of which
amounted to about 10% of the heat-supply. The calorimeter C, fig. 8,
was gilded, and completely surrounded by a nickel-plated steel
enclosure B, forming the bulb of a mercury thermo-regulator, immersed
in a large water-bath maintained at a constant temperature. In spite
of the large corrections the results were extremely consistent, and
the value of the temperature-coefficient of the diminution of the
specific heat of water, deduced from the observed variation in the
rate of rise at different points of the range 15 deg. to 25 deg.,
agreed with the value subsequently deduced from Rowland's experiments
over the same range, when his thermometers were reduced to the same
scale. Griffiths' final result for the average value of the calorie
over this range was 4.192 joules, taking the E.M.F. of the Clark cell
at 15 deg. C. to be 1.4342 volts. The difference from Rowland's value,
4.181, could be explained by supposing the E.M.F. of the Clark cells
to have in reality been 1.4323 volts, or about 2 millivolts less than
the value assumed. Griffiths subsequently applied the same method to
the measurement of the specific heat of aniline, and the latent heat
of vaporization of benzene and water.
[Illustration: FIG. 8.]
S 13. _Schuster and Gannon._--The method employed by A. Schuster and
W. Gannon for the determination of the specific heat of water in terms
of the international electric units (_Phil. Trans._ A, 1895, p. 415)
corresponded to the expression ECT, and differed in many essential
details from that of Griffiths. The current through a platinoid
resistance of about 31 ohms in a calorimeter containing 1500 grammes
of water was regulated so that the potential difference on its
terminals was equal to that of twenty Board of Trade Clark cells in
series. The duration of an experiment was about ten minutes, and the
product of the mean current and the time, namely CT, was measured by
the weight of silver deposited in a voltameter, which amounted to
about 0.56 gramme. The uncertainty due to the correction for the water
equivalent was minimized by making it small (about 27 grammes) in
comparison with the water weight. The correction for external loss was
reduced by employing a small rise of temperature (only 2.22 deg.), and
making the rate of heat-supply relatively rapid, nearly 24 watts. The
platinoid coil was insulated from the water by shellac varnish. The
wire had a length of 760 cms., and the potential difference on its
terminals was nearly 30 volts. The rate of stirring adopted was so
slow that the heat generated by it could be neglected. The result
found was 4.191 joules per calorie at 19 deg. C. This agrees very well
with Griffiths considering the difficulty of measuring so small a rise
of temperature at 2 deg. with a mercury thermometer. Admitting that the
electro-chemical equivalent of silver increases with the age of the
solution, a fact subsequently discovered, and that the E.M.F. of the
Clark cell is probably less than 1.4340 volts (the value assumed by
Schuster and Gannon), there is no difficulty in reconciling the result
with that of Rowland.
S 14. _H.L. Callendar and H.T. Barnes_ (_Brit. Assoc. Reports_, 1897
and 1899) adopted an entirely different method of calorimetry, as well
as a different method of electrical measurement. A steady current of
liquid, Q grammes per second, of specific heat, Js joules per degree,
flowing through a fine tube, A B, fig. 9, is heated by a steady
electric current during its passage through the tube, and the
difference of temperature d[theta] between the inflowing and the
outflowing liquid is measured by a single reading with a delicate pair
of differential platinum thermometers at A and B. The difference of
potential E between the ends of the tube, and the electric current C
through it, are measured on an accurately calibrated potentiometer, in
terms of a Clark cell and a standard resistance. If hd[theta] is the
radiation loss in watts we have the equation,
EC = JsQd[theta] + hd[theta] (2).
[Illustration: FIG. 9.]
The advantage of this method is that all the conditions are steady, so
that the observations can be pushed to the limit of accuracy and
sensitiveness of the apparatus. The water equivalent of the
calorimeter is immaterial, since there is no appreciable change of
temperature. The heat-loss can be reduced to a minimum by enclosing
the flow-tube in a hermetically sealed glass vacuum jacket. Stirring
is effected by causing the water to circulate spirally round the bulbs
of the thermometers and the heating conductor as indicated in the
figure. The conditions can be very easily varied through a wide range.
The heat-loss hd[theta] is determined and eliminated by varying the
flow of liquid and the electric current simultaneously, in such a
manner as to secure approximately the same rise of temperature for two
or more widely different values of the flow of liquid. An example
taken from the _Electrician_, September 1897, of one of the earliest
experiments by this method on the specific heat of mercury will make
the method clearer. The flow-tube was about 1 metre long and 1 millim.
in diameter, coiled in a short spiral inside the vacuum jacket. The
outside of the vacuum jacket was immersed in a water jacket at a
steady temperature equal to that of the inflowing mercury.