,350 SEC. 10. ELECTRICITY. 



There are us many teeth in the escapement wheel as there are letters on 

 the revolving disc ; the latter moves from one letter to the following for each 

 current sent. 



The train of wheel work is actuated by a mainspring. 



1549. Cooke and Wheatstone's Pointer Telegraph In- 

 strument, 1840. 



This instrument is similar in all respects to the revolving disc telegraph, 

 excepting that a pointer takes the place of the revolving disc. 



10 5O. Cooke and Wheatstone's Magneto-Electric Com- 

 municator, used with their revolving disc or pointer 

 alphabetical telegraph, 1840. 



This communicator is so arranged that a current is sent when its spoked 

 wheel is turned through a distance equal to that dividing the letters engraved 

 upon it. 



The commutator fixed on the axle of the revolving electro-magnet is so 

 constructed that the magneto-electric currents are all in the same direction. 



1516. Old Form of Double-Needle Instrument, with 

 six-inch coils. 



1512. Double-Needle Instrument, newer form. 

 Used by the Electric Telegraph Company. 



1514. Portable Double-Needle Instrument, which can 

 also be used for testing wires. 



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



1515. Model Double-Needle Instrument with a Key 

 Commutator, 1849. 



1511. Single-Needle Instrument, modern form, with 

 handle commutator. 



Used by the Electric Telegraph Company and by railways. 



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



used by the Electric Telegraph Company. 



a. The original form. 6-inch coils. 



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



c. Clark's Needle. 



d. S. A. Varley's coil with soft iron needle magnetised 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 



Avhich the Blackwall Railway was first worked, about 1840. 



