CHAP, in.] ELECTRIC TELEGRAPHY. 567 



order ; the whole takes place on the other side if the correspondence 

 is to be between Sevres and Versailles. 



Suppose now the stations at Paris and Versailles wish to corre- 

 spond directly. The sending station sends to Sevres the name of the 

 station to which the message is to go, and allows the necessary number 

 of minutes to elapse. The clerk at Sevres replies 00 (compris), and 

 then he puts the commutators on the bar for direct communication, 

 CD. All correspondence is stopped for this station during the whole 

 time the message is passing, a time which the motion of the galvano- 

 meters suffice to indicate. The message passed, the clerk replaces 

 the commutators on the contacts of the alarums. 



V. DIAL TELEGRAPHS (continued). 



There are several systems of dial or letter-showing telegraphs, but 

 practically they are reduced to two, namely the Siemens and Halske 

 system and the Wheatstone system. These two systems are based 

 upon the successive step by step development of the telegraph over a 

 series of years. Two of the more important of the early step by step 

 letter-showing telegraphs, those of Wheatstone in 1840 and Nott and 

 Gamble in 1846, are figured below. In Wheatstone's, the successive 

 letters forming the word appeared at the distant station at an opening 

 in the dial plate; the communicator dial of the instrument is furnished 

 with an alphabet, and the rotation of this dial bringing the required 

 letter to the zero, sends into the circuit the necessary succession of 

 " make and break " currents to cause a similar step by step rotation 

 of the distant indicating dial, by which means the required letter is 

 brought to view. 



In Nott and Gamble's dial telegraph, Fig, 366, the respective letters 

 or numerals were indicated by the step by step motion of a revolving 

 pointer, the necessary letter being indicated and controlled by succes- 

 sive " make and break " contacts with a battery by means of a finger 

 key and mercury cell h. Two electro-magnets a and d, acting upon 

 soft iron armatures in connection with a " Clawker" and driver motion, 

 rotated the toothed wheel c and external pointer. The electro-magnet 

 b controlled the alarum or call signal. 



The Siemens and Halske letter-showing telegraph, Fig. 367, is chiefly 



