90 ELEMENTS OF LABORATORY WORK 



61. Formation of an Electric Current when Two Dif- 

 ferent Metals are placed in Contact at each Extremity, and 

 the Two Junctions maintained at Different Temperatures, 

 Copper and iron wires, joined together and connected at their 

 free ends with a galvanometer, will also serve to exhibit a 

 relation between thermal changes and electric changes. If 

 the junction is now warmed by any hot body, such as a flame, 

 the magnet of the galvanometer will move. If the junction 

 is cooled, the magnet will move in the opposite direction. 



The simplest system would be a loop as shown (fig. 29, a), 

 but the introduction of a galvanometer or electrometer into 

 the circuit is needed to show the electric change produced by 

 the change of temperature. The whole of the galvanometer 

 coil may be considered as forming part of the copper wire, in 



GALVANOMETER. 



Fig. 29. 



which case the two junctions will be at A and B as shown 

 (fig. 29, b). 



A system of this kind resembles the systems previously 

 investigated, except that the cell is now absent. In the place 

 of the cell, and fulfilling its function, we have a hot body 

 and a junction of two kinds of matter. 



The greater the difference of temperature maintained be- 

 tween the two junctions, the greater will be the electric stress 

 produced up to a certain limit. 



The important correlation between electric change and 

 thermal change is illustrated in another manner. 



If the temperature of junctions in a heterogeneous circuit 

 containing a cell be carefully observed, it will be found that 

 at some parts there is a fall of temperature, at others a rise. 

 The direction of the change, however, is always the same for 



