THE ELECTRIC TELEGRAPH. 



therefore confine our observations to those apparatus which have 

 been actually employed on the telegraphic lines established in 

 different countries, and a very few others which appear to claim 

 more especial attention. 



On the claims of various projectors on the score of original 

 invention, we must generally decline to enter. To discuss such 

 questions so fully as to render justice to the claimants would 

 require much more space than we can devote to the subject ; and 

 however interesting such a discussion might be to the inventors 

 themselves and their partisans, it would offer but few attractions 

 to the masses to whom our " Museum" is addressed. 



We shall therefore first explain briefly the forms of telegraph 

 generally applied in this country, and next those which are in 

 operation elsewhere. 



179. The telegraphic instruments used almost exclusively in 

 this country are galvanometers (138), which make their signals 

 by means of the deflections of magnetic needles, produced by the 

 electric current. 



These instruments are of 'two forms, the first, and most simple, 

 consisting of one needle with its appendages and accessories, and 

 the other of two independent needles, each accompanied by its 

 own appendages. 



THE SINGLE NEEDLE INSTKUMENT. 



180. This instrument consists of a galvanometer and a commu- 

 tator, mounted in a case resembling in form and size that of an 

 ordinary table time-piece. 



A front view of it is given in fig. 66 (vol. iii. p. 161). On the 

 upper part is a dial, in the centre of which the indicating needle 

 appears, like the hand of a clock, fixed upon an axis. Its plaj r 

 to the right and left is limited by two ivory studs inserted in the 

 face of the dial, a short distance on each side of its upper arm. 



The handle which works the commutator, also fixed upon an 

 axis, is presented at the lower part of the case, under the dial. 



Upon the dial are engraved the letters of the alphabet, the ten 

 numerals, and one or two arbitrary symbols, under each of which 

 is engraved a mark, indicating the motions of the needle, by 

 which the letter or figure is expressed. 



The galvanometer, constructed as already explained (140), is 

 attached to the back of the dial, the axis of its magnetic needle 

 passing through the dial and carrying the indicating needle in 

 front. 



The latter is also usually magnetic, its poles being reversed in 

 their direction with relation to those of the interior needle, the 

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