THE ELECTRIC TELEGRAPH. 



course of the current, and those of hydrogen moving with it, the 

 former are evolved at the points o, and the latter at the points h. 



170. To show how this property of the current may be made to 

 produce visible marks or signs, let us suppose a sheet of paper 

 wetted with an acidulated solution of ferro-prussiate of potash 

 to be laid upon a plate of metal, and let the point of a metallic 

 style be applied to it so as to press it gently against the metallic 

 plate without piercing it. Let the style be now put in metallic 

 connection with the wire which leads to the positive pole of a 

 voltaic battery, and let the metallic plate upon which the paper is 

 laid be put in connection with the wire which leads to the 

 negative pole. The current will, therefore, flow from the style 

 through the moistened paper to the metallic plate, and it will 

 decompose the prussiate, one of the constituents of which deposited 

 on the paper will mark it with a bluish spot. 



If the paper be moved under the style while the current flows, 

 this decomposition being continued under the point of the style a 

 bluish line will be traced upon the paper. 



If while the paper is thus moved uniformly under the style, 

 the current is permitted to flow only during intervals long 

 or short, the paper will be marked by lines long or short, 

 according to the intervals during which the current flows ; and, 

 since no decomposition takes place during the suspension of the 

 current, the paper then passes under the style without receiving 

 any mark. If- the current be permitted to flow only for an 

 instant, the paper will be marked by a dot. The long or short 

 lines and dots, thus traced upon the paper, will be separated one 

 from another by spaces more or less wide according to the lengths 

 of the intervals of suspension of the current. 



It is evident that the same effects will be produced, whether 

 the style be at rest and the paper moved under it as is here sup- 

 posed, or the paper be at rest and the style moved over it. 



171. The paper may be moved under the style by various and 

 obvious mechanical expedients. Thus it may be coiled upon a 

 cylinder or roller, which being kept in constant and uniform 

 revolution by clock-work or other means, the paper would be 

 carried continually under the style, and unrolled from the cylinder 

 after receiving the marks. Or the cylinder covered with paper 

 might, while it revolves, receive a slow motion in the direc- 

 tion of its axis, so that the course of the style upon it would 

 be that of the thread of a screw or helix. The paper might be 

 cut into the form of a large circular disc, and laid upon a metallic 

 disc of equal magnitude, to which a motion of revolution round 

 its centre in its own plane might be imparted by clock-work ; 

 while the style might receive a slow motion directed from the 



