SECT. XXX. ROTATION BY ELECTRICITY. 313 



explained by Dr. Roget in some excellent articles on these sub- 

 jects in the Library of Useful Knowledge. 



All experiments tend to prove that the force emanating from 

 the electric current, which produces such effects on the magnetic 

 needle, acts at right angles to the current. The action of an 

 electrical current upon either pole of a magnet has no tendency 

 to cause the pole to approach or recede, but to rotate about it. If 

 the stream of electricity be supposed to pass through the centre of 

 a circle whose plane is perpendicular to the current, the direction of 

 the force exerted by the electricity will always be in the tangent to 

 the circle, or at right angles to its radius (N. 223). Consequently, 

 the tangential force of the electricity has a tendency to make the 

 pole of a magnet move in a circle round the wire of the battery. 



Rotatory motion was suggested by Dr. Wollaston. Dr. Fara- 

 day was the first who actually succeeded in making the pole of a 

 magnet rotate about a vertical conducting wire. In order to 

 limit the action of the electricity to one pole, about 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 surface, a current of positive elec- 

 tricity was made to descend perpendicularly through a wire 

 touching the mercury, and immediately the magnet began to 

 rotate from left to right about the wire. The force being uni- 

 form, the rotation was accelerated till the tangential force was 

 balanced by the resistance of the mercury, when it became con- 

 stant. Under the same circumstances the south pole of the 

 magnet rotates from right to left. It is evident, from this ex- 

 periment, 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 accomplished 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 revolved 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, re- 

 volve about their own axes, which they do with great rapidity. 



P 



