CHAP, in.] ELECTRIC TELEGRAPHY. 573 



VI. WHEATSTONE'S MAGNETO-ALPHABETICAL TELEGRAPH. 



One of the most perfect forms of the step by step letter-showing 

 telegraph is Wheatstone's magneto-A.B.C. instrument, which has since 

 1860 come into general use for short private wire telegraphs between 

 offices and works. The apparatus consists of three distinct parts, the 

 "communicator" for sending the message; the " indicator " for re- 

 ceiving the message, and the " alarum " for calling attention. 



The " communicator " consists of a powerful horse-shoe magnet, 

 with four coils of insulated wire wound round soft iron cores attached 

 to the poles. A soft iron armature is made to revolve rapidly in front 

 of the soft iron cores of the coils by means of a handle and multiplying 

 wheels, so that by the successive " make and break " of the revolving 

 armature before the poles of the magnet, currents of magneto-electricity 

 in alternate directions are rapidly generated, and in readiness to be' 

 passed into the line wire in any consecutive number of alternating 

 currents which may be necessary to indicate a signal. Over the magnet 

 is a fixed dial furnished with the letters of the alphabet, and other 

 signs arranged round in equal spacing. Finger buttons attached to 

 lever-keys are placed round the dial, each button being opposite to, 

 and corresponding to a letter or sign ; by a simple mechanical 

 contrivance, a circular slack chain is placed in connection with the 

 levers of the buttons, so that when any key is depressed by the finger 

 it draws up the slack and remains down while the rest of the chain 

 being tightened elevates the lever of the previously depressed button. 

 If no key is depressed down, the alternate currents developed by the 

 rapid " make and break " of the revolving armature before the poles 

 of the magnet, will pass continuously into the line wire, but if a 

 button key be depressed, the end of the key-lever will be thrown 

 forward, and arrest the revolution of a rotating arm set in motion by 

 the gearing of the armature, and the flow of the alternating currents 

 into the line wire will be cut off. Thus by the successive depression 

 of the necessary buttons by the finger, each button as depressed raising 

 the one previously depressed and releasing the rotating arm, alternating 

 currents flow into the line until the arm is again arrested by a de- 

 pressed button, and the number of these alternating currents corresponds 



