312 ELECTRO-MAGNETISM. SECT. XXX. 



SECTION XXX. 



Discovery of Electro-magnetism Deflection of the Magnetic Needle by a 

 Current of Electricity Direction of the Force Rotatory Motion by 

 Electricity Rotation of a Wire and a Magnet Rotation of a Magnet 

 about its Axis Of Mercury and Water Electro-Magnetic Cylinder 

 or Helix Suspension of a Needle in a Helix Electro-Magnetic Induc- 

 tion Temporary Magnets The Galvanometer. 



THE disturbing effects of the aurora and lightning on the mariner's 

 compass had been long known. In the year 1819 M. Oersted, 

 Professor of Natural Philosophy at Copenhagen, discovered that 

 a current of Voltaic electricity exerts a powerful influence on a 

 magnetized needle. This observation has given rise to the theory 

 of electro-magnetism one of the most interesting sciences of 

 modern times, whether it be considered as leading us a step farther 

 in generalization, by identifying two agencies hitherto referred to 

 different causes, or as developing a new force, unparalleled in the 

 system of the world, which, overcoming the retardation from 

 friction, and the obstacle of a resisting medium, maintains a 

 perpetual motion as long as the action of a Voltaic battery is 

 continued. 



When the two poles of a Voltaic battery are connected by a 

 metallic wire, so as to complete a circuit, the electricity flows 

 without ceasing. If a straight portion of that wire be placed 

 parallel to, and horizontally above, a magnetized needle at rest in 

 the magnetic meridian, but freely poised like the mariner's com- 

 pass, the action of the electric current flowing through the wire 

 will instantly cause the needle to change its position. Its 

 extremity will deviate from the north towards the east or west, 

 according to the direction in which the current is flowing ; and, 

 on reversing the direction of the current, the motion of the needle 

 will be reversed also. The numerous experiments that have been 

 made on magnetism and electricity, as well as those on the 

 various relative motions of a magnetic needle under the influence 

 of galvanic electricity, arising from all possible positions of the 

 conducting wire, and every direction of the Voltaic current, 

 together with all the other phenomena of electro-magnetism, are 



