346 



ELEMENTARY LESSONS ON [CHAP. vm. 



cold, hot lead brought into contact with cold lead shows 

 no perceptible difference of potential. 



LEAD 



-10 



-15 



o 



100 



200 



300 



400 



Fig. 143- 



An example will illustrate the usefulness of the diagram. Let 

 a circuit be made by uniting at both ends a piece of iron and a 

 piece of copper ; and let the two junctions be kept at o and 

 1 00 respectively by melting ice and boiling water. Then the 

 total electromotive-force round the circuit is represented by the 

 area a, o, -15, b. The slope of the lines for the various metals 

 represents the property referred to above, of an electromotive- 

 force between differently heated portions of the same metal 

 accompanied by an absorption or evolution of heat when the 

 current flows from a hotter to a colder portion of the same 

 metal. This effect, known as the Thomson effect from its 

 discoverer Sir W. Thomson, is opposite in iron to what it is 

 in copper or zinc. In copper, when a current of electricity flows 

 from a hot to a cold point, it evolves heat in the copper, and it 

 absorbs heat when it flows from a cold point to a hot point in 

 the copper. In iron a current flowing from a hot point to a 

 cold point absorbs heat. 



384. Thermo-electric Piles. In order to increase 



