518 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



secondary apparatus, such as the alarums or warning apparatus, 

 relays, and lightning conductors. They will be described in their place. 



Such are the principles of electric telegraphy, as it has been prac- 

 tised up to the present time. The number of systems which have 

 been and still are in use in the universal network is very large. We 

 can only propose to describe those in most general use ; and among 

 these the most original, that is, those which are distinguished by some 

 characteristic idea, by a special mechanism, or a particular method of 

 signalling. From this latter point of view we may class the electric 

 telegraphs in use under five groups. 



1st. The needle telegraphs. These have the indicators composed 

 of magnetised needles under the immediate action of the current 

 which circulates in a coil, which causes deflections to the right or left, 

 which are the elements of the signal. 



2nd. The dial telegraphs, in which the indicator consists of a dial 

 with an indicating needle whose motion is regulated by an electro- 

 magnet, under the action of a current alternately sent through the line 

 and interrupted. 



3rd. The writing telegraphs where the message sent is traced by 

 the indicator on a band of paper which unrols itself continuously ; 

 the signs which are stamped or marked in ink are produced by a style, 

 whose motion is due to the passage or interruption of the current. 



4th. The printing telegraphs, where the message itself is printed 

 in typographic characters, no translation being any longer necessary. 



5th. Autographic telegraphs which reproduce not only the text 

 but the facsimile even of the writing of the message, so that 

 signatures arid drawings may be sent and reproduced in the original 

 form. These apparatus have received for this reason the name of 

 pantelegraphs (from the Greek irav all). 



We pass now to the details of the mechanism of the principal 

 systems of telegraphy just enumerated. 



III. NEEDLE TELEGRAPHS. 



We commence with the needle telegraphs, which, as we have seen 

 above, are those which first received the sanction of serious and 

 practical experience. 



It is to Wheatstone that their invention is due. 



