XI. APPLICATIONS. 357 



Apparatus used by the Post Office in 1876. 



1561. Wheatstone's ABC Instrument, Communicator, 

 and Indicator. 



This instrument is used on wires leased to private persons, and for tha 

 least important postal lines. It is well adapted for these purposes on account 

 of not requiring skilled labour for its manipulation. The instrument is 

 worked by a magneto-electric machine within the instrument. 



Switches used with this instrument. 



Alarum 



1560. Single Needle Instrument. 



Used on lines of a class superior to those on which Wheatstone's A B C is 

 employed. It is but little liable to derangement, and requiring no adjustment 

 is preferred to recording or acoustic apparatus for circuits of secondary 

 importance. 



1558. Siemens 9 Direct Writing Morse Inker, with 

 Morse Signalling Key and Galvanometer. 



This instrument is well known, being used in almost every country ; its 

 advantage over the Morse embosser consists in the greater legibility of the 

 signals, which can be read in any light, and it requires much less power to 

 work it. This form is used on lines of moderate length, where the direct line 

 current is sufficient to actuate the electro-magnet without the assistance of a 

 relay and local battery. 



1559. Siemens' Local Ink-Writer. 



Used in connexion with a relay and local battery on lines where the direct 

 current is not sufficiently strong to actuate the direct ink-writer. 



1565. Sounder. 



This instrument is of American origin ; it is used instead of the ink-writer on 

 some of the lines of the Postal Telegraph Department, the dots and dashes of 

 the Morse alphabet being read by sound from the clicking of the armature 

 lever instead of from the paper band of the recording instrument ; it possesses 

 the advantage of not fatiguing the eye of the operator, and allows him perfect 

 freedom for writing the message. 



1562. Double Current Morse Key. 



It can be arranged either to be worked as a single current Morse key, or to 

 send double currents, as desired. 



1564a. Double-current Key, with levers instead of the 

 ordinary springs. 



These keys send double currents or reversals. By the marking current the 

 tongue of the distant relay is moved to the battery contact, by the reversed or 

 spacing current it is brought back to its rest stop. This arrangement in- 

 creases the range of adjustment, and as a current passes to line between the 

 signals the effect of leakage from other wires is in great measure neutralised. 



A switch, or a zinc-sender (1536), is required to alter the connexions when 

 changing from sending to receiving. 



