COMMUNICATION BETWEEN TWO STATIONS. 725 



ing bell-shaped supports of porcelain or glass, fixed upon 

 posts or against the sides of buildings. Each of the 

 electro-magnets is in conducting communication with 

 the axis on which the lever of the communicator or 

 ' sending key ' turns, and, in the ordinary position of the 

 key, it is also in connection, through the contact studs 

 at the farther end (b and d, fig. 364), with the earth 

 and with one pole of the battery, while the second 

 pole of the battery is connected by a wire with the 

 stud, with which the key makes contact when pressed 

 down. This is the state of matters shown in fig. 365, 

 and in this state, when the keys at both stations are 

 up, no current is transmitted, because only one pole of 

 each battery is connected with the circuit. Now let 

 the key at station I. be pressed down. The pole pre- 

 viously free is now connected by means of the front 

 contact studs with the lever of the key ; accordingly 

 the current passes along the lever to the axis, thence 

 through the spiral of the electro-magnet at station I., 

 along II to station II. and the electro-magnet there, 

 through the key at station II. to the plate e 2 and then 

 through the earth, and so back to the other pole of the 

 battery. A corresponding effect is produced if ^ re- 

 mains at rest while T 2 is pressed down at station II. 

 In general, whenever one of the two keys is pressed 

 down, a complete circuit is established, the armatures 

 of both electro-magnets are attracted, and the signals des- 

 patched at one station are simultaneously formed on the 

 moving paper at both stations. To save paper, however, 

 the clockwork is only set going at the receiving station, 

 not at that from which the message is being sent. The 

 attention of the clerk at the receiving station S 2 is 



