722 



TELEGRAPHY. 



FIG. 2. 



P M. Its construction is such that, when not 

 in use, one pole of the battery, the positive, 

 for example, is in connection with the line, 

 and the negative with the earth, necessitating 

 the passage of the current through the line in 

 the first place ; but when the key is touched 

 the negative pole is connected " to line " and 

 the positive to earth, reversing the direction 

 of the current. These reversals of direction 

 operate, as has been said, the polarized mag- 

 net P M. 



To revert to the illustration we made use of 

 in describing the duplex, let the reader picture 



to himself a water-course in 

 which both the direction and 

 the volume of the current can 

 be changed at pleasure. He 

 can suppose, in addition to the 

 water-wheels before figured, 

 and which will indicate the 

 force of the stream, a pair of 

 hinged valves or gates, which, 

 whether the current be strong 

 or weak, will be moved only 

 by a change in its direction. 

 The former will represent the 

 ordinary magnets, and the lat- 

 ter the polarized magnets. 



It is plain that, so far, this 

 is only another form of du- 

 plex, sending two messages 

 in the same direction at once. 

 To make it a quadruplex telegraph it is neces- 

 sary, in the first place, to add to it Stearns's 

 duplex, or a contrivance similar to it. Even 

 then a dead-lock would happen when the cur- 

 rents sent from each end of the line should be 

 of the same intensity, and opposite in direc- 

 tion; that is, when all eight operators were 

 working together. To remedy this, extra bat- 

 teries are introduced, which are neutralized by 

 part of the current in the main circuit, when 

 that is in a working condition, but are set free 

 to work the instruments when the currents in 

 the main circuit destroy one another. In the 



Eirth 



FIG. 3. MORSE KEY AND REGISTER. 



diagram, Fig. 2, the extra batteries, etc., have 

 been omitted, as also the transmitting appa- 

 ratus of one station and the recording instru- 

 ments of the other. 



The American Automatic system may be 

 said to have been perfected in 1873. The great 

 rapidity with which messages are transmitted 

 and recorded by it is its principal advantage, 



but it has others as requiring a smaller force 

 of operators and less specially skilled. The 

 usual work of a Morse operator is acknowledged 

 to be about 1,500 words an hour, but, by the 

 automatic method, to receive and print double 

 that number of words per minute is an or- 

 dinary feat, and as many as 7,000 words have 

 been legibly recorded in that time. As every 



