THE CALORIMETER SYSTEM AND MEASUREMENT OF HEAT. 1 17 



are very small, and by constantly heating or cooling the air-space next 

 the zinc, as indicated by the positive and negative deflections on the 

 galvanometer, the temperature of the zinc itself is increased or dimin- 

 ished, as the case may be, so that the temperature of the two metal walls 

 shall always be very nearly alike. 



The arrangements for heating and cooling the air-space next the 

 zinc wall are next to be considered. 



HEATING AND COOLING THE AIR-SPACK. 



The air-space surrounding the zinc wall and between this and the 

 inner wooden casing has been described (p. 1 1 1 ) , and it has been shown 

 that this space is divided by narrow strips into four sections, corre- 

 sponding with the top and bottom of the chamber and the upper and 

 lower halves of the walls. (See fig. 23.) Comparing this with the ex- 

 planation in the paragraphs j ust above, it will be seen that these divisions 

 of the air-spaces correspond with the areas covered by the different 

 groups of elements into which the whole system of thermo-electric 

 elements is subdivided. The devices here described for heating and 

 cooling the air-spaces are arranged so that the different sections of the 

 space may be heated or cooled independently. In other words, pro- 

 vision is made for determining temperature differences in different sec- 

 tions of the surface area of the calorimeter, and also for heating or 

 cooling the corresponding areas in the zinc wall as may be indicated. 

 Thus, it is possible to heat one space and cool the adjoining space at 

 the same time. 



Heating circuits. For heating the air-spaces a current of electricity 

 is passed through a circuit of German-silver wire, each separate space 

 having its individual heating circuit. The wire is threaded through 

 porcelain rings at the ends of each wooden panel and wound once 

 around porcelain knobs at three different points in the length of the 

 panel. By this arrangement the wire is firmly held in place, and even 

 during the slight sagging due to expansion when heated, is prevented 

 from coming in contact with the metal wall of the chamber. The 

 white porcelain knobs or insulators on which the heating wires are 

 strung are readily seen in figures 25, 29, 30, and especially in figure 31. 



The wire itself is so fine that it is hardly discernible in some of the 

 figures, being quite plainly seen, however, in figure 30. In the upper 

 and lower side spaces the current flows around all four sides at the 

 same time. The wires are attached to the wooden walls, however, in 

 such manner that they may be disconnected at the corners when the 

 panels are to be removed. The heating circuits for the top and bot- 

 tom sections are attached in one unbroken wire stretched continuously 

 back and forth across each of the respective panels. 



