XI. APPLICATIONS. 355 



of Cooke and Wheatstone's instrument, but was never brought into practical 

 use. 



[This apparatus was described by Gumming in 1827, in his " Electrodyna- 

 mics "; it is also mentioned in the Encyclopedia JSritannica, 7th ed., Art. 

 "Voltaic Electricity," and .in the treatise on " Eleciromagnetism " in the 

 Library of Useful Knowledge, 1832.] 



1543. Wheatstone's Type Printing Instrument, 1841. 



Steel type punches are fixed at the extremities of separate radiating springs 

 placed round the circumference of a horizontal wheel, which is fixed on the 

 quickest wheel of the train of wheel work, and governed by a dead beat 

 escapement actuated by electro-magnets. The paper band passes under the 

 type, and the printing is performed by an electro-magnet which causes a 

 hammer to strike the proper punch when it is opposite the paper. Alternate 

 layers of white and blackened paper are employed to receive the impressions 

 of the punch. 



1544. Theiler's Synchronous Type Printer, 1854. 



Two currents are required for each letter, one to start the instrument and 

 another to print. The type-wheel returns to zero after the printing of each 

 letter. 



1546. Theiler's " Step by Step" Type Printer, 1863-64. 



The type-wheel fixed on the axle of the escapement is controlled by re- 

 versals, and the printing is performed by the wheelwork, which is brought 

 into action by an electro-magnet, the local battery circuit of the latter being 

 closed by a vibratory arrangement, which does not act until the type-wheel is 

 stopped at the letter required. 



1547. Dujardin's " Step by Step " Type Printer, 1865. 



A step-by-step instrument in which the escapement of the. type-wheel is 

 controlled by reversals, the electro-magnets acting on the anchor of the 

 escapement being worked by a polarised relay in the line circuit, and a local 

 battery. 



The operation of printing is performed by an electro-magnet, the local 

 battery circuit of which is closed by the anchor of the escapement at the end 

 of each oscillation. 



When the apparatus is running, and no key is depressed, the currents 

 passing through the' printing magnet are too short to cause it to attract its 

 armature ; but when a key is depressed and the type-wheel stopped at any par- 

 ticular letter the duration of the current is lengthened, the printing magnet 

 attracts its armature, and an impression is made. . 



An electro-magnetic " cut off" arrangement is used in connexion with the 

 printing magnet, which causes the printing current to be equal in duration 

 whether the key be held down for a longer or shorter time, - 



This instrument was used for a short period on a wire" between London and 

 Edinburgh in 1865, by the Electric Telegraph Company. 



1545. Hughes' Type Printing Instrument. 



This is a purely synchronous instrument. The instruments at each end of 

 the line run at the same speed. 



There are as many keys on the key-board as there are letters on the type- 

 wheel, and the keys are so arranged that by pressing any one of them a 

 current is sent to line at the moment the letter it represents on the type-wheel 



Z 2 



