146 PHYSIOLOGICAL PHYSICS. ich.ip.xiv 



Measurements of temperature can be effected 

 with the aid of such apparatus as has been described. 

 So sensitive can the thermopile and galvanometer be 

 made that an elevation of temperature of O'OOOT of a 

 degree can be accurately estimated. This is owing to 

 the law that the electromotive force of the thermal 

 current is proportional to the difference of temperature 

 between the two junctions of the pile. Suppose, there- 

 fore, it is required to estimate the difference of tem- 

 perature between two bodies. They are placed in 

 contact jwith the two sides of the pile, and a deflection 

 of the needle is obtained. This deflection is proportional 

 to the difference of temperature of the two junctions, 

 and if the galvanometer has been previously graduated, 

 the difference can be easily calculated. The previous 

 graduation can be effected by keeping one set of 

 junctions of a pile at a constant temperature, and 

 varying the temperature of the other set. A note 

 can then be made of the extent of deflection corre- 

 sponding to certain differences. If it is desired to 

 determine the absolute temperature of a body, it is 

 necessary to place one side of the pile in contact with 

 some standard (say, melting ice), some body whose tem- 

 perature is known and is constant, and then place the 

 other side in contact with the body whose temperature 

 is to be measured. The difference of temperature 

 gives the absolute temperature by relation to the 

 standard. 



It is found, however, that with high temperatures 

 the deflection becomes so great that the proportion 

 between it and the difference of temperature is lost. 

 To maintain the proportion, a less sensitive instrument, 

 such as the tangent galvanometer (page 96), may be 

 used, or the current may be short-circuited by a rheo- 

 cord, and only a proportion of it made use of. 



To determine differences of temperature of dif- 

 ferent parts of body (say, of two sides of the body in. 



