SECT. XXX. GALVANOMETER. 315 



which these currents possess of inducing a particular state upon 

 matter in their immediate neighbourhood, otherwise neutral or 

 indifferent. For example, the connecting wire of a galvanic 

 battery holds iron filings suspended like a magnet as long as the 

 current continues to flow through it : the iron becomes magnetic 

 by the induction of the current. The most powerful temporary 

 magnets are obtained by bending a thick cylinder of soft iron into 

 the form of a horseshoe, and surrounding it with a coil of thick 

 copper wire covered with silk to prevent communication between 

 its coils. When this wire forms part of a galvanic circuit 

 the iron becomes so highly magnetic by the induction of 

 the current flowing through the wire that a temporary magnet 

 of this kind made by Professor Henry of the Albany Academy 

 in the United States sustained a weight of nearly a ton. Another 

 by Mr. Gage has Jbeen applied with considerable success as a 

 moving power : its spark is a bright flash, and the snap as loud as 

 a pistol. But the most powerful known is that employed lay Mr. 

 Joule in his experiments, which sustains a weight of 2080 Ibs. The 

 iron loses its magnetism the instant the electricity ceases to flow,and 

 acquires it again as instantaneously when the circuit is renewed. 



The action of an electric current causes a deviation of the 

 compass from the plane of the magnetic meridian. In proportion 

 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 equilibrium between 

 these two forces takes place, will indicate the intensity of the 

 galvanic current. The galvanometer, constructed upon this prin- 

 ciple, is employed to measure the intensity of galvanic currents 

 collected and conveyed to it by wires. This instrument is rendered 

 much more sensible by neutralizing the effects of the earth's mag- 

 netism on the needle, which is accomplished by placing a second 

 magnetised needle so as to counteract the action of the earth on the 

 first a precaution requisite in all delicate magnetical experiments. 



It has been ascertained by means of this instrument that the 

 action of an electrical current upon a magnet is inversely as 

 the square of the distance, and the energy with which an electro 

 magnet acts is directly as the power of the galvanic battery and 

 the number of coils round the core, and inversely as the resist- 

 ance of the wire. 



p 2 



