450 MAGNETIC EFFECTS OF 



It was first remarked by Davy, that the mag- 

 netic influence of a voltaic current was in pro- 

 portion to the heat developed in the wire con- 

 necting the extremities of the battery. (Philoso- 

 phical Transactions, 1821.) And it is now uni- 

 versally known, that batteries composed of a few 

 large plates of zinc and copper, or even of a 

 single pair of plates, are the best adapted for 

 exhibiting magnetic phenomena in fine, that 

 the magnetic power of voltaic electricity is pro- 

 portional to the quantity evolved, and inversely 

 as its tension, or power of communicating a 

 shock that thermo-electricity which affords 

 none of the usual signs of common electricity, 

 such as diverging the gold leaves of an electro- 

 meter, and passing rapidly through conductors, 

 deflects a delicate magnetic needle. It was long 

 since observed by M. CErsted, that common 

 electricity passed too rapidly through metallic 

 wires to affect the needle an hypothesis which 

 has been confirmed by M. Colladon of Geneva, 

 who found that when the wire was covered with 

 three folds of silk, (which must have retarded the 

 velocity of the electric current through it,) the 

 needle was deflected as by a current of voltaic 

 electricity. 



The most important application of (Ersted's 

 discovery was the invention of what has been 

 termed an electric multiplier, or galvanometer, by 

 M. Schweigger of Halle, which has been variously 

 modified by other philosophers. Before the in- 



