136 A RESPIRATION CALORIMETER. 



phosphor-bronze suspension wire of the instrument is still intact after 

 eight years' use. 



The galvanometer is placed in a black-cloth hood, shown in figure 

 37, and a straight filament, i6-candlepower lamp (a so-called bung-hole 

 lamp) used as a source of illumination, the image of the filament being 

 reflected on a millimeter scale immediately before the observer. 



The variations in resistance of the copper coils can be measured in 

 two distinct ways. In the one case it is possible to adjust a very deli- 

 cate variable resistance so that by comparing the resistances of the 

 copper coils with a Wheatstone bridge the slight variations in resist- 

 ance due to temperature fluctuations can be expressed in fractions of 

 an ohm. The second method depends upon the fact that the deflec- 

 tions on the galvanometer are very nearly proportional to the amount 

 of current passing through it, and consequently slight variations in the 

 current caused by slight alterations in resistance will produce corre- 

 sponding alterations in the deflection of the galvanometer. The amount 

 of current passing through the galvanometer is a function of two vari- 

 ables electro motive force and resistance. If the electro-motive force 

 is maintained constant, any alterations in resistance of the copper ther- 

 mometer coils, through which the current must flow when passing 

 through the galvanometer, will result in variations in the amplitude of 

 the galvanometer deflection. 



The first method of temperature measurements, i. c. , the use of the 

 slide- wire Wheatstone bridge, was followed entirely in the earlier form of 

 respiration calorimeter, 1 but the long-continued use of the slide-wire 

 bridge is open to serious objections. Temperature measurements in 

 experiments with the respiration calorimeter are made at intervals of 

 not more than 4 minutes, and frequently the experiments continue from 

 10 to 13 days. The constant wear and tear of the sliding contact on a 

 bridge of this type is a factor that must be taken into account in the 

 most accurate work, and accordingly we have devised an apparatus for 

 indicating temperature changes on the second of the two plans outlined 

 above. This apparatus is described in detail on pages 139-150. 



OBSERVER'S TAB^E. 



The various devices concerned in the regulation of the temperature 

 of the calorimeter and the measurement of heat are controlled from the 

 observer's table at the front of the apparatus. Figure 37 gives a gen- 

 eral view of the table and the adjacent apparatus. 



The wires from the systems of thermal junctions in the metal walls and 

 the surrounding air-spaces, and those from the resistance thermometers 



'U. S. Dept. of Agr., Office of Experiment Stations Bull. 63, pp. 25-27. 



