Polytechnic Association. 941 



through, long submarine cables, is that known as the mirror-galvano- 

 meter, by which a spot of light is made to follow, in its movements, 

 every variation in that force ; and these varying strengths being fol- 

 lowed by the eye, are thus interpreted under circumstances in which 

 ordinary telegraphic instruments indicating the mere presence or 

 absence of a given current would be useless. The author of this mir- 

 ror-galvanometer, Sir William Thompson, has invented another instru- 

 ment that records the transmitted message. It is now in practical 

 operation at the terminus of the French cable at Duxbury, Mass. His 

 instrument draws or marks on a strip of paper a curved or broken 

 line, giving graphic representation of the varying strength of the cur- 

 rent ; or, if required, of electric potential at the receiving end of the 

 cable, and gives a permanent record of every feature of the effect due 

 to the sender's action. The instrument is available for the system of 

 varying strength, including, of course, the simplest system, viz.: of 

 two strengths called positive and negative, or for the system of long 

 and short signals, of which the Morse alphabet is the simplest type. 

 The difficulty in producing such a recorder as this has been due to 

 the difficulty of obtaining marks from a very light body in rapid 

 motion without impeding that motion. To effect this, the inventor 

 connects (either by direct attachment, or by stretched thread or fiber) 

 to the body moved by the received current, a light markiiig-needle or 

 tube, from the end of which ink or other fluid is spurted upon paper. 

 The signals which are to be recorded give rise to motions of the 

 marking end, which are parallel to the plane of the pap'er, while the 

 paper is drawn along its own plane, and in a direction perpendicular 

 to the line of the motions caused by signals. He employs for the 

 marking-needle, by preference, a capillary tube, or a bristle, dipping 

 at one end into a stationary reservoir of ink or other fluid, and he 

 causes such fluid to be spurted from, or drawn from, the opposite end, 

 by means of an electric force, or by means of rapid vibrations main- 

 tained in the needle or in the paper, in a direction perpendicular to 

 the plane of the paper. These vibrations may be maintained mechan- 

 ically or pneumatically, as by the agency of sound, so that the paper 

 receives ink by a succession of fine contacts, between each of which 

 the tube or bristle. is quite free to move. When the electric method 

 is used the paper is drawn over a metal plate, electrified, say, posi- 

 tively, and the capillary tube negatively, and a powerful difference of 

 potential is maintained between the tube and the metal plate, such as 

 would tend to cause a succession of sparks to pass between them, and 

 which, in the circumstances, produce a fine stream of ink, or a succes- 



