SECT, xxxi.] ROTATION BY ELECTEICITY. 357 



two-thirds of a small magnet were immersed in mercury, the 

 lower end being fastened by a thread to the bottom of the 

 vessel containing the mercury. When the magnet was thus 

 floating almost vertically with its north pole above the sur- 

 face, a current of positive electricity was made to descend 

 perpendicularly through a wire touching the nfercury, and 

 immediately the magnet began to rotate from left to right 

 about the wire. The force being uniform, the rotation was 

 aecelerated till the tangential force was balanced by the 

 resistance of the mercury, when it became constant. Under 

 the same circumstances the south pole of the magnet rotates 

 from right to left. It is evident, from this experiment, that 

 the wire may also be made to perform a rotation round the 

 magnet, since the action of the current of electricity on the 

 pole of the magnet must necessarily be accompanied by a 

 corresponding reaction of the pole of the magnet on the 

 electricity in the wire. This experiment has been accom- 

 plished by a vast number of contrivances, and even a small 

 battery, consisting of two plates, has performed the rotation. 

 Dr. Faraday produced both motions at the same time in a 

 vessel containing mercury; the wire and the magnet re- 

 volved in one direction about a common centre of motion, 

 each following the other. 



The next step was to make a magnet, and also a cylinder, 

 revolve about their own axes, which they do with great 

 rapidity. Mercury has been made to rotate by means of 

 Voltaic electricity, and Professor Ritchie has exhibited in the 

 Royal Institution the singular spectacle of the rotation of 

 water by the same means, while the vessel containing it 

 remained stationary. The water was in a hollow double 

 cylinder of glass, and, on being made the conductor of elec- 

 tricity, was observed to revolve in a regular vortex, changing 

 its direction as the poles of the battery were alternately 

 reversed. Professor Ritchie found that all the different con- 

 ductors hitherto tried by him, such as water, charcoal, &c., 

 give the same electro-magnetic results when transmitting the 



