MISCELLANEOUS PRACTICAL APPLICATIONS. 327 



each station responds to the reversing key at the other station, 

 and the neutral relay at each station responds to the key at the 

 other station which alters the strength of the current. 



33. The printing telegraph is an arrangement by means of which 

 a simple form of typewriter is operated at a distant station from a 

 keyboard at a sending station. A simple form of printing tele- 

 graph is as follows : * Twenty -six equidistant pins are arranged 

 in a helical row around a long metal cylinder. This cylinder is 

 rotated by a small electric motor or by clockwork, and above the 

 cylinder is a bank of twenty-six lettered keys so arranged that 

 when a key is depressed, one of the pins comes against it and the 

 cylinder is stopped in a certain position ; the next key would 

 stop the cylinder ^g ^ a revolution farther on, and so on. At- 

 tached to the rotating cylinder is a device for reversing an elec- 

 tric current fifty-two times for each revolution of the cylinder. 

 This repeatedly reversed electric current passes over the tele- 

 graph line and through two electromagnets at the receiving 

 station. One of these electromagnets is like a neutral relay with 

 a heavy lever, and the other is like a polarized relay with a light 

 lever which oscillates with the rapid reversals of current and 

 actuates an escapement which turns a type wheel with the twenty- 

 six letters arranged round its periphery. This type wheel is thus 

 turned step by step, keeping pace with the rotating cylinder at 

 the sending station. 



When the cylinder at the sending station is stopped by de- 

 pressing a key, the A-key, for example, the current-reversing 

 device stops also, a steady current flows over the line, the tongue 

 of the polarized relay stops oscillating, the type wheel stops, and 



* When a person is thoroughly familiar with the elements which enter into the con- 

 struction of a machine, that is, when a person is familiar with shafts and wheels, and 

 with simple devices like switches for opening and closing electric circuits and for re- 

 versing connections, a more easily intelligible description of a complicated machine 

 can be made without illustrative diagrams and drawings than can be made with the 

 help of diagrams and drawings. In fact, it is confusing under the specified conditions 

 to have recourse, even, to a working model of a complicated machine, when the object 

 in view is to impart a clear idea of its fundamental features. 



