The measurement of the temperature differences of the water-current by
the electric-resistance thermometer was tried a number of years ago by
Rosa,[10] but the results were not invariably satisfactory and in all
the subsequent experimenting the resistance thermometer could not be
used with satisfaction. More recently, plans were made to incorporate
some of the results of the rapidly accumulating experience in the use of
resistance thermometers and consequently an electric-resistance
thermometer was devised to meet the conditions of experimentation with
the respiration calorimeter by Dr. E. F. Northrup, of the Leeds &
Northrup Company, of Philadelphia. The conditions to be met were that
the thermometers should take rapidly the temperature of the ingoing and
outcoming water and that the fluctuations in temperature difference as
measured by the resistance thermometers should be controlled for
calibration purposes by the differences in temperature of the mercurial
thermometers.
[Illustration: FIG. 16.--Details of resistance thermometers for
water-circuit. Upper part of figure shows a sketch of the outside of the
hard-rubber case. In lower part is a section showing interior
construction. Flattened lead tube wound about central brass tube
contains the resistance wire. A is enlarged part of the case forming a
chamber for the mercury bulb. Arrows indicate direction of flow on
resistance thermometer for ingoing water.]
For the resistance thermometer, Dr. Northrup has used, instead of
copper, pure nickel wire, which has a much higher resistance and thus
enables a much greater total resistance to be inclosed in a given space.
The insulated nickel wire is wound in a flattened spiral and then passed
through a thin lead tube flattened somewhat. This lead tube is then
wound around a central core and the flattened portions attached at such
an angle that the water passing through the tubes has a tendency to be
directed away from the center and against the outer wall, thus insuring
a mixing of the water. Space is left for the insertion of the mercurial
thermometer. With the thermometer for the ingoing water, it was found
necessary to extend the bulb somewhat beyond the resistance coil, so
that the water might be thoroughly mixed before reaching the bulb and
thus insure a steady temperature. Thus it was found necessary to enlarge
the chamber A (fig. 16) somewhat and the tube leading out of the
thermometer, so that the bulb of the thermometer itself could be placed
almost directly at the opening of the exit tube. Under these conditions
perfect mixing of water and constancy of temperature were obtained.
In the case of the thermometer which measured the outcoming water, the
difficulty was not so great, as the outcoming water is somewhat nearer
the temperature of the chamber, and the water as it leaves the
thermometer passes first over the mercurial thermometer and then over
the resistance thermometer. By means of a long series of tests it was
found possible to adjust these resistance thermometers so that the
variations in resistance were in direct proportion to the temperature
changes noted on the mercurial thermometers. Obviously, these
differences in resistance of the two thermometers can be measured
directly with the Wheatstone bridge, but, what is more satisfactory,
they are measured and recorded directly on a special type of automatic
recorder described beyond.