316 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



the electromotive force of the element is a function of the tern* 

 perature T of the other junction, and, if the electromotive force 

 of the element is determined once for all for a series of values of 

 T, then any unknown temperature may be determined by observ- 

 ing the electromotive force of the thermo-element when one of 

 its junctions is at the standard temperature and the other is at the 

 temperature which is to be measured. 



The electromotive force of a thermo-element can * be repre- 

 sented with a fair degree of accuracy by the equation 



(i) 



when one junction of the element is kept at a fixed standard tem- 

 perature, a, b and c being constants. Therefore in order to 

 use a thermo-element as a pyrometer, it is sufficient to measure the 

 electromotive force e of the element for three chosen known 

 values of T. The thermo-element which has proved most satis- 

 factory for use as a pyrometer is orie employing pure platinum 

 and an alloy of platinum and rhodium. 



The Peltier effect. In 1834 Peltier discovered that heat (in- 

 dependently of the heat generated in accordance with Joule's 

 Law, Art. 12, Chapter II) is generated or absorbed at a junc- 

 tion of two metals when a current flows across the junction, that is, 

 heat is generated when the current flows in one direction and 

 absorbed when the direction of the current is reversed ; the gen- 

 eration of heat being shown by an increase of temperature of the 

 junction, and the absorption of heat being shown by a cooling of 

 the junction. For strong currents this Peltier effect is masked 

 by the heat that is generated on account of electrical resistance, 

 for the rate of generation of heat by the Peltier effect is proportional 

 to the current, while the rate of generation of heat on account of 

 resistance is proportional to the square of the current. The 

 Peltier effect is most easily shown as follows : A current from a 

 voltaic cell is sent through a thermopile. This current heats one 

 set of junctions and cools the other set. The thermopile is then 



* See Magnetism and Electricity for Students, H. E. Hadley, pages 361-367. 



