Fig. 240.
Silvering and plating by the magneto machine, turned by a steam-engine.
The magneto-electric telegraph patented by Mr. Henley in 1848, offers another example of the application of the electric current induced in electro-magnetic coils, when they rotate in close proximity to the poles of a powerful steel magnet. This telegraph is now in constant use by the English and Irish Magnetic Telegraph Company, through a distance of more than 2100 miles. The whole length of wires in use amounts to the astonishing quantity of 13,900 miles, of which 6350 miles are hidden underground, and 7500 conducted above.
This telegraph is considered to be one of the simplest and most economical yet brought into practical working.
CHAPTER XX.
DIA-MAGNETISM.
At the end of the chapter devoted to the subject of light, will be found an experiment devised and carried out by Dr. Faraday, in which it is shown that if a bar of a peculiar glass (called after the inventor, Faraday's heavy glass, or silicated borate of lead) is subjected to the inductive action of a very powerful electro-magnet, that it has the power of changing the direction of a ray of polarized light transmitted through it. This effect is not confined to the poles of an electro-magnet, but is also perceptible (though in a diminished degree) with ordinary magnets.
The result of this important experiment was communicated to the Royal Society by Dr. Faraday on the 27th November, 1845, the enunciation of the fact by this learned philosopher being, "that when 'the line of magnetic force' is made to pass through certain transparent bodies parallel to a ray of polarized light traversing the same body, the ray of polarized light experiences a rotation." Now, "the line of magnetic force" means that continual flow of the magnetic current which passes from pole to pole, and is indicated by iron filings sprinkled on paper placed above the poles of a magnet, and usually termed magnetic curves, or the curved lines of magnetic force. (Fig. 241.)