368 IDENTITY OF MAGNETISM [SECT. xxxm. 



the helices which concentrate it ; and, while one set conveys 

 a current to the disc, the other set conducts the opposite cur- 

 rent to the copper slip. As the edge of the revolving disc 

 is always immersed in the mercury, one set of wires is con- 

 stantly maintained in contact with it, and the circuit is only 

 completed when a point of the copper slip dips in the mer- 

 cury also ; but the circuit is broken the moment that point 

 rises above it. Thus, by the rotation of the armature, the 

 circuit is alternately broken and renewed ; and, as it is only 

 at these moments that electric action is manifested, a bril- 

 liant spark takes place every time the copper point leaves 

 the surface of the mercury. Platina wire is ignited, shocks 

 smart enough to be disagreeable are given, and water is de- 

 composed with astonishing rapidity, by the same means; 

 which proves, beyond a doubt, the identity of the magnetic 

 and electric agencies, and places Dr. Faraday, whose ex- 

 periments established the principle, in the first rank of ex- 

 perimental philosophers. 



By electrifying and magnetizing a ray of polarized light 

 Dr. Faraday has given a new and elegant instance of the 

 identity of these two powers. If a ray of polarized light, 

 and the direction of the electric force of an electro-magnet 

 be parallel, or nearly parallel, while passing through a piece 

 of flint glass, or silico-borate of lead placed between the poles 

 of the electro-magnet, the ray, viewed through a piece of 

 tourmaline, or a Nicholas eye-piece, will vanish and reap- 

 pear as usual at each quarter revolution of the eye-piece ; 

 but, if at the time the ray has vanished a current of elec- 

 tricity be sent through the electro-magnet, the light will 

 instantly reappear, and, as often as the electric current 

 flows or is interrupted, so often does the polarized ray ap- 

 pear and vanish. The same phenomena were produced with 

 a common horse-shoe magnet, so that the action of elec- 

 tricity and magnetism is the same on polarized light, but it 

 requires electro-magnets and magnets of great power to 

 show it. 



