ELECTRICITY. 109 



are so numerous and so variable, that it is difficult to arrive 

 at definite results. Thus the relative size of the coil and the 

 iron, the temper or degree of hardness of the latter, its shape, 

 or the proportions of length to diameter, the number of coils 

 surrounding it, the conducting power of the metal of which 

 the coils are formed, the size of the keeper or iron in which 

 magnetism is induced, the degree of constancy of the bat- 

 tery, &c., complicate the experiments. 



The most trustworthy general relation which has been as- 

 certained is, that the magnetic attraction is as the square of 

 the electric force ; a result due to the researches of Lenz and 

 Jacobi, and also of Sir W. S. Harris. 



Lastly, electricity produces chemical affinity ; and by its 

 agency we are enabled to obtain effects of analysis or synthe- 

 sis with which ordinary chemistry does not furnish us. Of 

 these effects we have examples in the brilliant discoveries, by 

 Davy, of the alkaline metals, and in the peculiar crystalline 

 compounds made known by Crosse and Becquerel. 



