ELECTEO-MAGNETISM. 



431 



In what man- 

 ner does an 

 electric current 

 exert its mag- 

 netic furce ? 



Fig. 362. 



By liieans of the galvanometer, the most feeble traces of electricity can be 

 detected ; and electric currents which would fail to intiuence the most sensi- 

 tive gold leaf electrometer can be made to affect perceptibly the magnetic 

 needle. Galvanometers are sometimes called electro-multipUers. 



804. Electricity, unlike all other motive 

 forces in nature, exerts its magnetic force lat- 

 erally ; all other forces exerted between two 

 points act in the direction of a straight line 

 connecting their points, but the electric current exerts its 

 magnetic influence at right angles to the direction of its 

 course. 



When a magnetic pole is influenced by an electric cur- 

 rent, it does not move either directly toward or directly 

 from the conducting wire, but it tends to revolve about it. 



By the application of these facts, it has been discovered that rotatory move- 

 ments can be produced by magnets around conducting wires, and conversely, 

 that conducting wires can be made to rotate around magnets. 



The rotation of the pole of a magnet around a fixed conducting 

 wire may be shown by a piece of apparatus represented by Fig. 

 362. A small magnet, X, is fixed to the lower part of a vessel, 

 V, by means of a silk thread ; the vessel is filled with mercury 

 nearly to the top of the magnet; G is a conducting wire dipping into 

 the mercury, and Z is another conductor communicating with tho 

 mercury at the bottom of the vessel Now, when the electric 

 current is established, by connecting the extremities of the wires 

 C and Z ■^\-ith the opposite poles of the battery, the pole N of the 

 magnet revolves round the conducting wire C. If the current is 

 descending, that is, if C bo connected with the positive pole of 

 the battery, and if N be a north polo, its motion round the wire wiU'be di- 

 rect, that is, in the dhrection of the hamls of a watch ; and 

 BO on, vice versa, 



A different arrangement, by which a movable wire tra- 

 versed by a current, may be made to revolve around the 

 pole of a fixed magnet, is represented by Fig. 363. A wire, 

 A B, is suspended from the wire C by a loop, and dips into 

 the mercury in a vessel, Y ; when the circuit is established, 

 ty connecting C and N with the respective poles of tho 

 battery, the conducting wire revolves around tho pole N 

 of the magnet. 



If the current be descending, and N be the north polo of 

 the magnet, the rotation will bs direct. 



On similar principles, various kinds of reciprocating and rotatory movementa 

 may be produced. 



Fig. 363. 



