WHAT IS ELECTRICITY'? — HEYL 219 



But it often happens that as soon as one theory is comfortably set- 

 tled on the throne another rises up to challenge its supremacy. We 

 shall see the reign of each successive theory of electricity growing 

 shorter. The thousands of years of the first era were followed by 

 three centuries of the second. In the first half of the nineteenth 

 century great things were happening. In 1820 Oersted had discov- 

 ered that an electric current could produce a magnetic effect, thus 

 tying together what had previously been regarded as separate phe- 

 nomena. In 1822 Seebeck showed that electricity could be generated 

 by heat. These discoveries impressed themselves on the mind of 

 Faraday, then at work in the Royal Institution. He was familiar 

 with the work of Davy in producing chemical decomposition by 

 electricity, and the converse phenomenon of Volta, the production 

 of electricity by chemical action. Faraday was also aware of the 

 converse of Seebeck's discovery, the production of heat (and light) 

 in the electric arc, and his thoughts turned naturally toward the 

 undiscovered converse of the Oersted effect. He says himself at a 

 later time ° (1845): 



I have long held an opinion, almost amounting to conviction, in common, I 

 believe, with many other lovers of natural knowledge, that the various forms 

 under which the forces of matter are made manifest have one common origin ; 

 or, in other words, are so directly related and mutually dependent, that they 

 are convertible, as it were, into one another, and possess equivalents of 

 power in their action. In modern times the proofs of their convertibility have 

 been accumulated to a very considerable extent, and a commencement made of 

 the determination of their equivalent forces. 



Such were the considerations which led Faraday to attempt the 

 generation of electricity by means of a magnet (1831). The story 

 is familiar to all of us; how he placed a magnet in a helix of wire 

 and found that no current was produced except momentarily while 

 the magnet was being placed in or taken out of the coil. This dis- 

 covery seems to have made quite an impression in other than scientific 

 circles, as is evidenced by some verse which has come down to us : 



Around the magnet, Faraday 



Is sure that Volta's lightnings play. 



To bring them out was his desire. 

 He took a lesson from the heart ; 

 'Tis when we meet, 'tis when we part, 



Breaks forth the hid electric fire. 



Encouraged by this success, Faraday later (1845) sought and 

 found a correlation between magnetism and light. Twenty years 

 later this in its turn furnished the inspiration for Maxwell's electro- 

 magnetic theory, by means of which the domain of optics was 

 annexed to that of electricity. 



* Faraday, Experimental Researches in Electricity, vol. 3, p. 1, Loudon, 1855. 



