ELECTRO-MAGNETISM. 



121 



other of these planes of revolution, depending on the direction of the revolving 

 current. 



On the 25th of the same month, Ampere communicated to the Academy 

 another paper.* In this he delivered the results of his experiments on the 

 reciprocal attractions and repulsions of electric currents acting on each other. 

 He showed that two straight wires, along which currents are transmitted will 

 attract or repel each other, according to the direction of the currents. Let a 

 line be imagined intersecting both wires at right angles. If both currents 

 move toward this perpendicular or both from it, the wires will attract each 

 other ; but if, while one of the currents moves toward this perpendicular, the 

 other moves from it, then they will repel each other. If the wires be parallel 

 to each other, they will attract or repel each other, according as the currents 

 move in the same or opposite directions. If the wires be in the same plane, 

 but not parallel, their directions will meet if produced : in this case they will 

 attract each other, if the currents be both directed toward or from the point 

 where their directions meet ; and they will repel each other, if one current be 

 directed toward, and the other from, that point. 



In the same paper he proposes the hypothesis of currents of electricity cir- 

 culating round the terrestrial globe, from east to west, in planes at right angles 

 to the direction of the dipping needle, to account for the phenomena of terres- 

 trial magnetism. 



These researches proceeded with unusual celerity. On the 9th of the fol- 

 lowing month (October), three weeks after the reading of the last-mentioned 

 paper, he presented another memoir to the Academy, in which he investigated 

 the properties of currents transmitted through wires forming closed curves 

 (courbes fermees), or complete geometrical figures. 



While Ampere was proceeding with these researches, Arago directed his 

 inquiries to the state of the wire through which the current was transmitted, 

 more especially with a view to determine whether every part of its surface 

 was endowed with the same magnetic properties. With this view he placed 

 iron filings within the sphere of attraction of the wire, and found that they ad- 

 hered to it, so as to form concentric rings upon it. The moment the connexion 

 of the wire with the pile was broken, and the current was no longer transmit- 

 ted along it, the filings fell off, and all attraction disappeared. 



By a process inferred from the theory of Ampere, M. Arago succeeded in 

 imparting permanent magnetism to needles and bars of steel by means of the 

 electric current. This was accomplished by making a spiral of wire, through 

 which the current was transmitted, while the needle or bar to be magnetized 

 was placed within its coils. The position of the poles of the magnets thus 

 made depended on the direction of the screw, or helix, formed by the conduct- 

 ing wire. If the wire formed a right-handed screw, the poles were placed in 

 one direction ; and if it made a left-handed screw, they were reversed. When 

 the wire was made to form a succession of screws alternately right-handed and 

 left-handed, the bar was magnetized with a corresponding series of consequent 

 points. The same results were obtained whether the electricity transmitted 

 through the wire proceeded from a Voltaic apparatus or from the common elec- 

 trical machine, f 



At the same time, or a very little later, and before the information of Arago's 

 experiments reached England, Davy succeeded also in imparting magnetism 

 to needles by the Voltaic current, and by common electricity ; and also showed 

 the effect of the conducting wire on iron filings.^ 



* Annales de Chimie et Physique, torn, xv., 59-170. 



t Annales de Chimie et Physique, torn, xv., p. 93. 



t Letter to Wollaston, 12th of November, 1820, Philosophical Transactions, 1821. 



