CHAPTER XII. 

 THE ELECTRICITY OF THE EARTH AND ATMOSPHERE. 



POTENTIAL AND EARTH CURRENTS. 



TERRESTRIAL and atmospheric electricity are so in- 

 timately related, that to obtain a correct knowledge of 

 either requires the consideration of its relations to the 

 other. 



Viewing electricity as a universal property of mat- 

 ter, its existence in the earth and atmosphere follows 

 as a necessary consequence. Hence, we are to study 

 its phenomena in this connection, rather than to ac- 

 count for its origin. These phenomena pertain chiefly 

 to difference of potential between different parts of the 

 earth's surface; different parts of the atmosphere; and 

 between the earth's surface and the atmosphere. 



This \lifference of potential results from various 

 causes. We have already seen how difference of po- 

 tential may be produced, artificially, by various instru- 

 ments, which are combinations of different substances, 

 having different degrees of electric resistance and con- 

 ductivity. By similar methods nature, on a grand 

 scale, produces results of which ours are but feeble im- 

 itations. 



ILLUSTRATIONS FROM THE THERMOPILE. In the 

 thermopile we have an illustration of the method by 

 which difference of potential is produced by heat. This 



