•1821.] Historical Sketch of Electro-magnetism. 279 



•exactly similar; and the view already given of them, as it relates 

 to straight wires, is consequently more simple than the descrip- 

 tion of the effects with spiral wires can be ; i. e. considering it 

 as a matter of experiment only, and not of theory. 



Inconsequence of the view which M. Ampere had taken of the 

 ■nature of magnetism as dependant simply upon currents of elec- 

 tricity, it became an important object with him to ascertain the 

 action of the earth upon such, currents excited by the voltaic 

 battery ; for from his theory (presently to be stated), he expected 

 that it would be equally efficient in directing these currents as 

 in directing those supposed to exist in the magnetic needle. 

 After some trials, he succeeded in overcoming the obstacles to 

 delicate suspension, contact, &c. and constructed an apparatus 

 in which a part of the wire connecting the two poles of a bat- 

 tery was rendered so light and mobile as to move immediately ; 

 the connection was completed with the pole, and took a direction 

 which, with regard to the earth, was always constant, and in 

 accordance with M. Ampere's theory. An account of these 

 experiments, with the apparatus used in them, was read to the 

 Royal Academy on Oct. 30. The first consisted of a wire bent 

 so as to form almost a complete circle of about 16 inches in 

 diameter; the two extremities were made to approach, and were 

 placed one just beneath the other ; and being attached to two 

 steel points, were connected by them with two little basins of 

 platina containing mercury, fixed so as to receive them; only one 

 of the points touched the bottom of the cup it was placed in; 

 so that the friction was scarcely any, and the mercury secured 

 a good contact. The cups were connected with other wires that 

 passed oft' to the voltaic battery; so that it was easy to make 

 this moveable circle connect either one way or the other between 

 the poles ; and being inclosed in a glass case, any movement it 

 might receive was readily observable without danger of its 

 resulting from any other cause than the electric action. 



When the extremities of this apparatus were connected with 

 the poles of a battery, the circle immediately moved, and after 

 some oscillations placed itself in a plane perpendicular to the 

 magnetic meridian of the earth; and on every repetition of the 

 experiment, the same effect took place. The direction in which 

 it moved depended upon the way in which the connexion had 

 been made with the battery ; and if it be assumed that there is 

 a current passing through the wire from the positive to the nega- 

 tive end, the curve so arranged itself that that current always 

 passed downwards on the eastern side, and upwards on the 

 west. This circle moved round a perpendicular, and, therefore, 

 only represented the direction of the magnetic needle: in order 

 lo represent the dip, a wire was formed into a. parallelogram, and 

 being fixed to a jjiass axis was suspended by line points, and 

 i oaneeted as before sd as to move round an horizontal axis; then 

 this axis being placed perpendicular to the magnetic meridian, 



