SCIENCE AND PRACTICE. 



375 



it possesses over the expansion-thermometers is that its 

 indications may be read off at almost any distance ; and by 

 a little ingenious contrivance the unknown temperature of 

 the leading wires will not cause the least error. This is 

 attained by connecting one end of the thermometer wire 

 with the outside casing of the instrument (or earth), the 

 other end with one of the leading wires, and the remaining 

 leading wire also with the casing. The leading wires are 

 made of the same metal, of the same length of course, and 



are adjusted to be of the same resistance. They are bound 

 up together so that they each must have the same tempera- 

 ture in any point. Thus arranged, the thermometer is put 

 in its place of rest, and the ends of the leading wires con- 

 nected with the measuring apparatus, which consists of a 

 "Wheatstone's balance. The resistances r and / (Fig. 173) 

 are equal to each other, of the same metal, and coiled 

 together upon a common reel. In the circuit of the leading 

 wire I, connected with the casing of the thermometer, an 

 adjustable resistance, R, is inserted. The whole then forms 

 a balance, giving us in the proportion of equilibrium, 



r' 



- 



I + B, 



or, as r =/ and / = V, 



Knowing beforehand the resistance of R' at C., its tern- 



