ELECTRO-MAGNETISM. 



but the polarity was reversed, taking alternately one direction or the other. 

 These alternations of intensity and polarity appeared to be determined in a 

 great measure by the weight, diameter, and conducting power of the wire, and 

 the strength of the electric discharge. 



One of the most novel and unexpected circumstances attending the experi- 

 ments of M. Savary, was the manner in which he showed that the magnetizing 

 influence of the current was modified by transmitting it through other metals. 

 When the needle to be magnetized was enveloped in metallic leaf, the magnet- 

 ism it received was augmented. By gradually increasing the thickness of 

 its metallic coating, the force of the magnetism it received increased by de- 

 grees till it attained a maximum, after which it diminished ; and, by further 

 augmenting the thickness of its coating, it was diminished so as to be equal to 

 the magnetism it would receive without any coating. Copper, tin, gold, silver, 

 and mercury, used as coating, were found to possess this property in different 

 degrees. The force of the electric charge transmitted through the wire was 

 found to have a singular influence on the phenomenon ; for, according as this 

 force was increased or diminished, different thicknesses of the same coating 

 were necessary to produce equal effects. These considerations also affected 

 the direction of the polarity. 



These facts appeared to M. Savary to be scarcely compatible with any hy- 

 pothesis which requires the admission or the translation of electric matter by 

 the current ; and he considered that they indicated rather that the current pro- 

 ceeds from a system of undulations propagated along the wire, and transmitted 

 by it laterally to adjacent media. 



THERMO-ELECTRICITY. 



The fact that a derangement of the equilibrium of temperature was attended 

 with the evolution of electric effects was observed by Volta, and subsequently 

 by Dessaignes. Volta found that a plate of silver, one end of which was more 

 heated than the other, produced Galvanic effects ; and Dessaignes observed 

 that convulsions were produced in the frog, when the muscles and nerves were 

 connected by a silver spoon in which lighted charcoal was placed. These 

 isolated observations, however, led to no conclusions affecting the progress of 

 discovery. 



Immediately after the discovery of Oersted became known throughout Eu- 

 rope, Professor Seebeck, of Berlin, engaged in a series of researches on the 

 Voltaic effects produced by derangement of temperature ; and communicated 

 to the Academy of Sciences of Berlin, during the years 1821 and 1822, the 

 results of his experiments, which were published in the " Transactions" of that 

 body, and form the basis of whatever has since been collected under the title 

 of thermo-electricity. 



A rod of copper being bent into a semicircle, a bar of antimony was soldered 

 to it, so that the two metals had the form of a stirrup. The temperature of one 

 of the points of junction of the metals was raised, while that of the other was 

 unchanged. An electric current was immediately excited, passing from the 

 copper at the heated point through the antimony. The intensity of the current 

 was augmented by augmenting the difference of temperature of the two points 

 of connexion of the metals, and the direction of the current was reversed 

 when the source of heat was removed from one point of junction to the oth- 

 er. The current was rendered manifest by its power to deflect a magnetic 

 needle. 



Seebeck observed similar effects by combining various other metals in pairs ; 

 and found that the relative electric state of the metals did not correspond with 



