by Electricitj/ in Motion. 285 



The first thing which we must determine, is the law accord- 

 ing to which the force emanating from the conjunctive wire 

 decreases at different distances from its axis. This inquiry has 

 been the object of experiments which I made along with 

 M. Savart, already known to the Academy by his ingenious dis- 

 coveries in Acoustics. We took a small needle of magnetized 

 steel in the form of a parallelogram, and to ensure its perfect 

 mobility, we suspended it under a glass bell, by a single fibre 

 of the silk-worm, and gave it at the same time a horizontal 

 direction. Then in order that it might be entirely at liberty to 

 obey the force emanating from the conjunctive wire, we screened 

 it from the action of the magnetism of the earth, by placing a 

 magnetized bar at such a distance, and in such a direction, that 

 it exactly balanced this action. Our needle was thereby placed 

 in the same freedom of movement as if there did not exist any 

 terrestrial globe, or as if we had been able to transport our- 

 selves with it to a great distance in space. We now presented 

 it to a conjunctive cylindrical wire of copper, stretched in a ver- 

 tical direction, and to which we had given such a length, that 

 its extremities necessarily bent in order to attach them to the 

 poles of the electric apparatus, should have, in consequence of 

 their distance, so feeble an action on the needle that it might 

 be neglected with impunity. This disposition represented 

 therefore the effect of an indefinite vertical wire, acting on a 

 horizontal and independent magnetic needle. As soon as the 

 communication of the voltaic current was completed, the needle 

 turned transversely to the axis of the wire, conformably to the 

 revolutive character indicated by M. Oersted ; and it set itself 

 to oscillate around this direction, as a clock pendulum, moved 

 from the perpendicular, oscillates round the vertical by the 

 effect of gravitation. We counted with an excellent seconds 

 watch of M. Breguet, the time in which a certain number of these 

 oscillations, twenty for example, were performed ; and by re- 

 peating this observation, when *[.e wire and needle were at dif- 

 ferent distances, we inferred the decreasing intensity of the force, 

 precisely as we determined by the oscillations of the same pen- 

 dulum, the variations of gravity at different latitudes. We thus 



