The mercurial thermometers used for measuring the temperature
differences of the water-current are of special construction and have
been calibrated with the greatest accuracy. As the water enters the
respiration chamber through a horizontal tube, the thermometers are so
constructed and so placed in the horizontal tubes through which the
water passes that the bulbs of the thermometers lie about in a plane
with the copper wall, thus taking the temperature of the water
immediately as it enters and as it leaves the chamber. For convenience
in reading, the stem of the thermometer is bent at right angles and the
graduations are placed on the upright part.
The thermometers are graduated from 0° to 12° C. or from 8° to 20° C.
and each degree is divided into fiftieths. Without the use of a lens it
is possible to read accurately to the hundredth of a degree. For
calibrating these thermometers a special arrangement is necessary. The
standards used consist of well-constructed metastatic thermometers of
the Beckmann type, made by C. Richter, of Berlin, and calibrated by the
Physikalische Technische Reichsanstalt. Furthermore, a standard
thermometer, graduated from 14° to 24° C., also made by Richter and
standardized by the Physikalische Technische Reichsanstalt, serves as a
basis for securing the absolute temperature. Since, however, on the
mercurial thermometers used in the water-current, differences in
temperature are required rather than absolute temperatures, it is
unnecessary, except in an approximate way, to standardize the
thermometers on the basis of absolute temperature. For calibrating the
thermometers, an ordinary wooden water-pail is provided with several
holes in the side near the bottom. One-hole rubber stoppers are inserted
in these holes and through these are placed the bulbs and stems of the
different thermometers which are to be calibrated. The upright portion
of the stem is held in a vertical position by a clamp. The pail is
filled with water, thereby insuring a large mass of water and slow
temperature fluctuations, and the water is stirred by means of an
electrically driven turbine stirrer.
The Beckmann thermometers, of which two are used, are so adjusted that
they overlap each other and thus allow a range of 8° to 14° C. without
resetting. For all temperatures above 14° C., the standard Richter
thermometer can be used directly. For temperatures at 8° C. or below, a
large funnel filled with cracked ice is placed with the stem dipping
into the water. As the ice melts, the cooling effect on the large mass
of water is sufficient to maintain the temperature constant and
compensate the heating effect of the surrounding room-air. The
thermometers are tapped and read as nearly simultaneously as possible. A
number of readings are taken at each point and the average readings used
in the calculations. Making due allowance for the corrections on the
Beckmann thermometers, the temperature differences can be determined to
less than 0.01° C. The data obtained from the calibrations are therefore
used for comparison and a table of corrections is prepared for each set
of thermometers used. It is especially important that these thermometers
be compared among themselves with great accuracy, since as used in the
calorimeter the temperature of the ingoing water is measured on one
thermometer and the temperature of the outgoing water on another.
Thermometers of this type are extremely fragile. The long angle with an
arm some 35 centimeters in length makes it difficult to handle them
without breakage, but they are extremely sensitive and accurate and have
given great satisfaction. The construction of the bulb is such, however,
that the slightest pressure on it raises the column of mercury very
perceptibly, and hence it is important in practical use to note the
influence of the pressure of the water upon the bulbs and make
corrections therefor. The influence of such pressure upon thermometers
used in an apparatus of this type was first pointed out by Armsby,[8]
and with high rates of flow, amounting to 1 liter or more per minute,
there may be a correction on these thermometers amounting to several
hundredths of a degree. We have found that, as installed at present,
with a rate of flow of less than 400 cubic centimeters per minute, there
is no correction for water pressure.
In installing a thermometer it is of the greatest importance that there
be no pressure against the side of the tube through which the
thermometer is inserted. The slightest pressure will cause considerable
rise in the mercury column. Special precautions must also be taken to
insulate the tube through which the water passes, as the passage of the
water along the tube does not insure ordinarily a thorough mixing, and
by moving the thermometer bulb from the center of the tube to a point
near the edge, the water, which at the edge may be somewhat warmer than
at the center, immediately affects the thermometer. By use of the vacuum
jacket mentioned above, this warming of the water has been avoided, and
in electric-resistance thermometers special precautions are taken not
only with regard to the relative position of the bulb of the mercury
thermometer and the resistance thermometer, but also with regard to the
hard-rubber insulation, to avoid errors of this nature.