SECT, xxxr.] ELECTKOMAGNETIC INDUCTION. 359 



suspended like an artificial magnet, as long as the current 

 continues to flow through it ; and the most powerful tempo- 

 rary magnets that have ever been made are obtained by 

 bending a thick cylinder of soft iron into the form of a 

 horse-shoe, and surrounding it with a coil of thick copper 

 wire covered with silk, to prevent communication between 

 its parts. When this wire forms part of a galvanic circuit, 

 the iron becomes so highly magnetic, that a temporary mag- 

 net of this kind, made by Professor Henry, of the Albany 

 Academy, in the United States, sustained nearly a ton weight. 

 The iron loses its magnetic power the instant the electricity 

 ceases to circulate, and acquires it again as instantaneously 

 when the circuit is renewed. Temporary magnets have 

 been made by Professor Moll of Utrecht, upon the same 

 principle, capable of supporting 200 pounds' weight, by 

 means of a battery of one plate less than half an inch square, 

 consisting of two metals soldered together. It is .truly 

 wonderful that an agent, evolved by so small an instrument, 

 and diffused through a large mass of iron, should communicate 

 a force which seems so disproportionate. Steel needles are 

 rendered permanently magnetic by electrical induction ; the 

 effect is produced in a moment, and as readily by juxtapo- 

 sition as by contact ; the nature of the poles depends upon 

 the direction of the current, and the intensity is proportional 

 to the quantity of electricity. 



It appears that the principle and characteristic phenomena 

 of the electro-magnetic science are, the evolution of a tangen- 

 tial and rotatory force exerted between a conducting body and 

 a magnet ; and the transverse induction of magnetism by the 

 conducting body in such substances as are susceptible of it. 



The action of an electric current causes a deviation of the 

 compass from the plane of the magnetic meridian. In pro- 

 portion as the needle recedes from the meridian, the intensity 

 of the force of terrestrial magnetism increases, while at the 

 same time the electro-magnetic force diminishes ; the number 

 of degrees at which the needle stops, showing where the 



