326 M. Arago on the influence of non-magnetic Bodies 



they have made the experiments on non-metallic bodies at too 

 great distances. 



The cause of the phenomena produced by metallic and other 

 bodies in rotation, has been generally attributed to the forma- 

 tion of a certain number of poles situated upon the non-mag- 

 netic body, and which, subsisting during a certain time, are 

 supposed to be sufficient either to fetter the motion of the 

 needle, when the disc remains immoveable, or to cause it to 

 turn in the case when the disc itself is put in motion. This 

 explanation, apparently so simple, is however liable to the ob- 

 jection that the formation of these poles, even if their existence is 

 admitted, would be insufficient to account for the motion of 

 the needle. If the observers who give this explanation had 

 endeavoured to compute the force which might be supposed to 

 reside in these poles, they would have found that the limit of 

 the motion which they could have communicated to the needle 

 would perhaps not have exceeded a minute, whilst, in order to 

 explain the rotation, it should have exceeded 90 degrees. 



Not content with this refutation of the common hypothesis, 

 M. Arago has endeavoured to point out its insufficiency by di- 

 rect experiments. 



Having suspended above the disc which he uses in his ex- 

 periments, a vertical magnetic needle, which can move only by 

 turning round its axis in a plane also vertical, and passing 

 through the radii of the disc ; and having put the disc in mo- 

 tion, he observed the needles carried towards the centre of the 

 disc, whenever it was placed at a less distance than about two- 

 thirds of the radius of the disc from its centre. At this distance, 

 the needle remained immoveable, while at a greater distance it 

 was carried in a contrary direction, or from the centre of the 

 disc. When the distance was equal to the radius, and even 

 greater, the needle was still pushed in the same direction. 



M. Arago next placed a needle in a horizontal situation, 

 so that it could move only round its middle in a horizontal 

 plane, and so that one of its extremities was found above and 

 very near the disc. When the disc was made to turn, this ex- 

 tremity of the needle was raised, as if it had been repelled 

 by it. 



As the force which is developed in a great number of cases 

 is repulsive between the different parts of the di?c and the 



