XI. APPLICATIONS. 301 



1514. Portable Double-Needle Instrument, which could 

 also be used for testing wires. 



Each of the needles worked by two finger keys behind the case. Used by 

 the Electric Telegraph Company. 



1515. Model Double-Needle Instrument with a Four- 

 Pedal Commutator, 1849. 



Model Double-Needle Instrument with a four-pedal commutator, 1849. 



1516. Old Form of Double-Needle Instrument. 



Old form of Double-Needle Instrument with six-inch coils and crutch 

 handles. 



1517. Series, showing the several Forms of Coil and Needle 



used by the Electric Telegraph Company. 



a. The original form. 6-inch coils. 



b. Holmes' diamond needle. 1-inch, 1848. 



c. Clark's Needle. 



d. S. A. Varley's coil with soft iron needle magnetized by induction. 



e. Spagnoletti's do. 

 /. Brittan's do. 



1518. Early Train Signalling Instruments. 



a. Cooke's first " block " instrument used on the Norfolk Railway, about 



1845. 



b. Step by step train indicator used on the London and South-western 



Railway. 



c. Do. do. South-eastern Railway. 



d. Signalling instrument used for starting and stopping the endless rope by 



which the Blackwall Railway was first worked, about 1840. 



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. 



