CONTRIBUTIONS TO SCIENCE. 529 



considerable current will flow through the wire. This current 

 Faraday called the extra- current. It is more generally 

 referred to as the self-induction current. 



A similar action takes place when connection is made 

 between a battery and a coil. The current does not at once 

 acquire its full value, but for a short time goes on steadily 

 increasing ; the self-induction of the coil causing it to behave 

 as if the current in it possessed considerable mass, which has 

 in the first instance to be put into motion. All these 

 actions are immediate consequences of the law of induced 

 currents stated on p. 526. 



There is a well-known experiment of Faraday in which a 

 specimen of his heavy glass, or borate of lead, was placed 

 between the poles of a powerful electro-magnet and a beam 

 of plane polarised light was passed through the glass in the 

 direction of the magnetic force. Faraday found that when 

 the light passed from the north to the south pole of the 

 magnet the plane of polarisation was turned through an 

 angle in the same direction as a right-handed screw would 

 rotate if piercing a solid and advancing with the light. 

 When the light passed in the opposite direction, the rotation 

 of the plane of polarisation was in the same direction with 

 respect to the magnet, and therefore reversed with respect 

 to the path of the light. In this respect the heavy glass 

 under the influence of the magnet behaved differently from 

 a solution of sugar which always turns the plane of polarisa- 

 tion of the light in the same direction with reference to its 

 direction of transmission. This was the first experiment 

 which showed any relation between light and magnetism, 

 and indicated that the medium which serves as the vehicle 

 of light the luminiferous ether must at least be affected 

 by the presence of magnetic force, though the fact that the 

 presence of ponderable matter is necessary to the production 

 of this rotation, and that the direction of the rotation depends 

 on the nature of the matter, renders it doubtful how far mag- 

 netic force affects the ether directly. 



All transparent solids and liquids exhibit the same action 

 on light in different degrees. If a tube of water with plate 



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