BAIN'S ELECTRO-CHEMICAL TELEGRAPH. 



Of the forms of telegraph in which this principle is brought into 

 play, the only one which has been practically applied on an 

 extensive scale is that projected by Mr, Alexander Bain. 



209. To render this instrument understood, let us suppose a 

 sheet of writing paper to be wetted with a solution of prussiate of 

 potash, to which a little nitric and hydrochloric acid have been 

 added. Let a metallic desk be provided corresponding in magni- 

 tude with the sheet of paper, and let this desk be put in com- 

 munication with a galvanic battery so as to form its negative pole. 

 Let a piece of steel or copper wire forming a pen be put in con- 

 nection with the same battery so as to form its positive pole. Let 

 the sheet of moistened paper be now laid upon the metallic desk, 

 and let the steel or copper point which forms the positive pole of 

 the battery be brought into contact with it. The galvanic circuit 

 being thus completed, the current will be established, the solution 

 with which the paper is wetted will be decomposed at the point 

 of contact, and a blue or brown spot will appear. If the pen be 

 now moved upon the paper, the continuous succession of spots will 

 form a blue or brown line, and the pen being moved in any 

 manner upon the paper, characters may be thus written upon it as 

 it were in blue or brown ink. 



An extremely feeble current is sufficient to produce this effect ; 

 but it will be necessary, when the strength of the current is very 

 much reduced, to move the pen more slowly, so as to give the 

 time necessary for the weakened current to produce the decom- 

 position. In short, a relation exists between the greatest speed 

 of the pen which is capable of leaving a mark, and the strength 

 of the current ; the stronger the current the more rapidly may 

 the pen be moved. In this manner, any kind of writing 

 may be inscribed upon the paper, and there is no other limit to 

 the celerity with which the characters may be written, save 

 the dexterity of the agent who moves the pen, and the sufficiency 

 of the current to produce the decomposition of the solution in 

 the time which the pen takes to move over a given space of the 

 paper. 



210. The electro-chemical pen, the prepared paper, and tbe 

 metallic desk being understood, we shall now proceed to explain 

 the manner in which a communication is written at the station 

 where it arrives. 



211. The metallic desk is a circular disk, about twenty inches 

 in diameter. It is fixed on a central axis, with which it is 

 capable of revolving in its own plane. An uniform movement of 

 rotation is imparted to it by means of a small roller, gently 

 pressed against its under surface, and having sufficient adhesion 

 with it to cause the movement of the disk by the revolution of 



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