Thermodynamic Properties of Air. 



313 



In this manner a platinum resistance is obtained of some 

 220 ohms at 0°. It forms one branch T (fig. 5) of a Wheat- 

 stone bridge. A second branch is formed by a resistance of 



Fisr. 5. 



german-silver wire contained in a case K of exactly the same 

 construction as shown on fig. 4, but on a larger scale ; it is 

 kept at the constant temperature 0° by melting ice. The 

 third and fourth branches of the Wheatstoiie bridge are resist- 

 ance-coils in one and the same box R, namely, a coil of 

 1000 ohms and a variable resistance R. B is a battery of 

 two elements of Leclanche, M a commutator, Gr a sensitive 

 galvanoscope. 



To every temperature of T there corresponds a determinate 

 resistance R in the box. By comparison with a normal 

 hydrogen-thermometer, it is possible to construct a diagram, 

 or a table, with the aid of which one may find by inspection 

 the temperature T corresponding to any observed resistance R. 

 Such a table does not cease to be true when the apparatus, 

 after having been dismounted, is to be used again several 

 days or months later ; the relation of T and R is of course 

 independent of the electromotive force of the battery or the 

 sensitiveness of the galvanoscope. 



A table of this kind is given below; at the same time it 

 shows the relation of resistance and temperature (hydrogen- 

 scale) for a certain platinum wire. 



