In order to detect differences in temperature between the copper and
zinc walls, some system for measuring temperature differences between
these walls is essential. For this purpose we have found nothing that is
as practical as the system of iron-German-silver thermo-electric
elements originally introduced in this type of calorimeter by E. B.
Rosa, of the National Bureau of Standards, formerly professor of physics
at Wesleyan University. In these calorimeters the same principle,
therefore, has been applied, and it is necessary here only to give the
details of such changes in the construction of the elements, their
mounting, and their insulation as have been made as a result of
experience in constructing these calorimeters. An element consisting of
four pairs of junctions is shown in place as T-J in fig. 25.
One ever-present difficulty with the older form of element was the
tendency for the German-silver wires to slip out of the slots in which
they had been vigorously crowded in the hard maple spool. In thus
slipping out of the slots they came in contact with the metal thimble in
the zinc wall and thus produced a ground. In constructing the new
elements four pairs of iron-German-silver thermal junctions were made on
essentially the same plan as that previously described,[6] the only
modification being made in the spool. While the ends of the junctions
nearest the copper are exposed to the air so as to take up most rapidly
the temperature of the copper, it is somewhat difficult to expose the
ends of the junctions nearest the zinc and at the same time avoid
short-circuiting. The best procedure is to extend the rock maple spool
which passes clear through the ferule in the zinc wall and cut a wide
slot in the spool so as to expose the junctions to the air nearest the
ferule. By so doing the danger to the unprotected ends of the junctions
is much less. The two lead-wires of German silver can be carried through
the end of the spool and thus allow the insulation to be made much more
satisfactorily. In these calorimeters free use of these thermal
junctions has been made. In the chair calorimeter there are on the top
16 elements consisting of four junctions each, on the rear 18, on the
front 8, and on the bottom 13. The distribution of the elements is made
with due reference to the direction in which the heat is most directly
radiated and conducted from the surface of the body.
While the original iron-German-silver junctions have been retained in
two of these calorimeters for the practical reason that a large number
of these elements had been constructed beforehand, we believe it will be
more advantageous to use the copper-constantin couple, which has a
thermo-electric force of 40 microvolts per degree as against the 25 of
the iron-German-silver couple. It is planned to install the
copper-constantin junctions in the calorimeters now building.