55(3 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



of the multiplier. According to M. Glcesener, this addition doubles 

 the power of Wheatstone's apparatus. 



Bain's single needle telegraph depends upon a different princi- 

 ple to those just described. The eldctro- magnetic agent is an electro- 

 magnet whose bobbins react on two permanent magnets in the form 

 of semicircles, movable about an axis which carries the indicating 



FIG. H55. Henley and Foster's magneto telegraph, 1848, Indicator movement. 



needle. The simultaneous attractions and repulsions in one direction 

 or the other produced in the poles of the electro-magnet and the 

 permanent magnets by the passage of the galvanic current deflect 

 the needle to the left or bring it back to its vertical position. The 



Fro. 356. Indicator of needle telegraph, Foy and Breguet's systeir.. 



manipulator is a simple commutator for reversing the poles, worked 

 by a handle, the latter being brought back to the vertical by springs. 

 Bain's telegraph used to work between Edinburgh and Glasgow from 

 1846 onwards. 



