CHAP, iv.] ELECTRIC TELEGRAPHY. 597 



current being passed into the circuit. The arrangement for supplying 

 the revolving disc with ink is simple. A metal wheel having its edge 

 cut into a V shape revolves in a reservoir of ink, and by capillary 

 attraction this V groove is kept filled with ink, so that the periphery 

 of the little inking disc, which revolves in this V groove of ink, is 

 kept constantly supplied without friction, and is thus enabled to 

 continuously record the rapid motion of the armature as the currents 

 flow into the line from the transmitting apparatus. 



IV. AUTOGRAPHIC TELEGRAPHS. CASELLI'S AND MEYER'S SYSTEM. 



We have seen that the idea of using the electrolytic properties of 

 a battery for transmitting signals dates from the earliest years of this 

 century. The names of Coxe, Soemmering and Schweigger are connected 

 with the first attempts. The signals were made in Soemmering's 

 telegraph by bubbles of hydrogen. In 1839 E. Davy made use of 

 electro-chemical reactions to print signals on a sheet of paper or cloth 

 suitably prepared. Twelve years later Bain constructed a writing 

 telegraph, based on the property of the galvanic current to decompose 

 cyanide of potassium and to produce a coloured compound, Prussian 

 blue, which is deposited on the paper of the indicator every time 

 the current passes and as long as it continues. The manipulating 

 apparatus as well as the indicator was the same as in Morse's. 

 Bain obtained on the band of paper blue points and marks of 

 greater or less length, whose combination furnished the elements of 

 the message. At first Bain made the metal pen of the indicator 

 describe a close set spiral 011 a sheet of ordinary paper, but the 

 principle was the same. 



Other electro-chemical telegraphs have been invented since, but we 

 cannot undertake to describe them. We will only consider those 

 systems or apparatus which are now known as autographic or pan- 

 telegraphs, and which have received the sanction of practical use. 



It is not the object in this new kind of printing telegraphs to 

 transmit signals which, like the writing telegraphs, may leave traces of 

 the message, or even reproduce it, and print it in alphabetical cha- 

 racters. The problem proposed, and solved with marvellous ingenuity, 

 is to obtain at the receiving station a faithful reproduction or true 



