XI. APPLICATIONS. 353 



The simplest form of relay, consisting of an electro-magnet ; its armature 

 is attached to one end of a lever, the other end playing between two limiting 

 stops, the local battery circuit being closed when the armature is attracted. 



1540. Earliest Form of Belay with an inducing 

 Magnet, used by the Electric Telegraph Company. 



The coil is wound on a reel of soft iron, upon each end of which a 

 hollow " casing " or cap of the same material is fitted, almost completely 

 encasing the coil in soft iron. The armature is shaped thus, 

 and is magnetised by induction from a compound bar magnet 

 placed behind. The crescent-shaped portion plays between the 

 inner ends of the casings, which for that purpose do not quite 

 meet, but leave the central portion of the coil exposed. 



An ordinary magnetic needle pivoted below the coil is acted upon by the 

 latter, and serves as an indicator to call attention. 



The armature is held up against knife-edge bearings by two helical springs, 

 and the adjustment is effected by varying the tension of one of them. 



1541. Later form of Relay, used by the Electric Telegraph 

 Company, 1856. A few are still in use by the Post Office. 



A horizontal bar of soft iron is pivoted vertically, and is free to move in 

 the interior of two cylindrical bobbins. The ends of the bar which project 

 beyond the bobbins play between the poles of horsehoe permanent magnets 

 fixed at each end. The relay is adjusted by moving the stops, and conse- 

 quently the soft iron bar, to one side or the other. 



1519. Henley's Magneto-electric Double-Needle In- 

 strument, 1848. Used by the British and Irish Magnetic Tele- 

 graph Company. 



The needles only move on one side of their vertical position, and the signals 

 are made up of the single and combined movements of the two needles. 



This instrument requires two line wires, and is worked by the magneto- 

 electric current generated by moving the handle or handles. 



The interior needles are small straight bar magnets, playing between the 

 semicircular pole pieces of an electro-magnet The needle remains on the 

 side on which it is left by the last current which passes through the coils, and 

 does not return to its vertical position by gravity, as in Cooke and Wheatstone's 

 needle instrument. 



1520. Henley's Magneto-electric Single-Needle Instru- 

 ment, 1848. Used by the British and Irish Magnetic Telegraph 

 Company. 



The dots and dashes of a modification of the Morse alphabet are repre- 

 sented by the duration of deflection of the needle on one side only of its normal 

 position. It is worked by the magneto-electric current generated by moving 

 the handle. Its construction is precisely similar in principle to Henley's mag- 

 neto-electric double-needle instrument. It was used on less important lines 

 than those on which the latter instrument was adopted. 



1521. Highton's Needle Telegraph, 1848. Used by the 

 British and Irish Magnetic Telegraph Company. 



A horseshoe or circular magnet within a circular coil worked by a 

 reversing key. 



40075. Z 



