176 ELEMENTS OF STATIC ELECTRICITY. 



instrument is a combination of metal bars, whose con- 

 ductivity for heat and electricity varies greatly. A 

 number of these bars, arranged in compact form, and 

 properly insulated, are soldered together in an alter- 

 nating series: so that a current of electricity, passing 

 through them, has to pass from one metal to the other. 

 They are folded together, and mounted in such a man- 

 ner, that heat may be applied to one set of junctions; 

 while the opposite, alternate set, is cooled. 



In this way, instruments are constructed, in which a 

 very slight difference of temperature, between the op- 

 posite sets of junctions, creates a perceptible difference 

 of electric potential: and powerful batteries are con- 

 structed in the same manner. 



The earth may be regarded as an immense battery of 

 this kind; being composed of heterogeneous materials, 

 whose conductivity for heat and electricity varies 

 greatly: and which are "subjected to great extremes of 

 temperature, at opposite junctions, fulfilling exactly 

 the conditions of the thermopile. 



The ocean, a vast, homogeneous conductor, is sepa- 

 rated into different parts by the great continents; whose 

 conductivity differs from it greatly: the five great 

 divisions of the ocean, and the two continents, con- 

 stituting an alternating series of conductors, of differ- 

 ent conductivities. 



The surface of the continents, composed of rock and 

 soil, of lakes, rivers, and sandy deserts, presents a great 

 diversity of material, of widely different conductivity. 



In the torrid and frigid zones, we have the opposite 

 extremes of temperature; which, in the thermo-electric 

 battery, are produced by exposing one set of junctions 

 to the heat of a lamp furnace ; while the opposite set is 



