1821.] Historical Sketch of Electro-magnetism. 197 



Whatever be the cause which is active within the connecting- 

 wire, whether it be the passage of matter through it, or the 

 induction of a particular state of its parts, it produces certain 

 very extraordinary effects. If small, it becomes heated ; and as 

 the size of the wire is diminished, or that of the apparatus 

 increased, the heat rises to an intense degree apparently without 

 any limitation, except from the influence of external circum- 

 stances, or the alteration of the wire. Another effect, and it is 

 that which has been discovered by M. Oersted, is, that, if brought 

 towards a magnetic needle, it has the power of attracting and 

 repelling it in a constant manner, and in obedience to certain 

 simple laws. 



If a magnetic needle be left to take its natural direction, and 

 then a straight portion of the connecting wire be brought above 

 it, and parallel to it, that end of the needle next the negative 

 pole of the battery moves towards the west ; and that whether 

 the wire be on the one or the other side of the needle, so that it 

 be above and parallel to it. If the connecting wire be sunk on 

 either side the needle so as to come into the horizontal plane in 

 which the needle is allowed to move, there is no motion of the 

 needle in that plane ; but the needle attempts to move in a ver- 

 tical circle; and, but for the imperfect suspension, and the 

 influence of the earth's magnetism, would do so. When the 

 wire is on the east of the needle, the pole of the needle next the 

 negative end of the battery is elevated; and when on the west of 

 the needle, it is depressed. If the connecting wire be now sunk 

 below the level of the needle, similar attractions and repulsions 

 take place, but in opposite directions to those followed when it is 

 above. The pole of the needle opposit e the negative end of the bat- 

 tery now moves eastwards, whatever the position of the wire, so 

 that it be restricted as above. 



That these positions of the magnetic needle may be re- 

 tained with more facility in the memory, Prof. Oersted proposes 

 the following formula: "the pole above which the negative electri- 

 city enters is turned to the west ; wider which, to the east." 



Oersted immediately pointed out, whnt it is easy to see from 

 the above experiments, that the movement of the needle took 

 place in a circle round the connecting wire ; and though in the 

 description of his first experiments the quantity of declination 

 given to the needle from the wire is expressed by an angle of so 

 many degrees, yet it is immediately stated to vary with the power 

 of t he battery. Whenever the needle is moved in a horizontal 

 or any other circle from the position it naturally assumes, the 

 power of the earth over it tends to restore that position, and is 

 consequently an active force in the present instance opposed to 

 the power of the connecting wire ; it, therefore, lessens the 

 declination the needle would otherwise have. Also when 

 the wire is brought into the same horizontal circle with the 

 needle, its cU'ect over it is shown by the elevation and denies- 



