as before at the relay, which, instead of being wound differentially, is known as a poor relay. Normally the currents going to the line flow from each station in such directions that they neutralize each other, while the other portions of the divided circuit flow over an artificial line and into the ground. Now when a signal is to be sent, the operator by means of a key or pole-changing instrument reverses the direction of the current, which passes through his relay without effect, but disturbs the conditions in that instrument at the distant station and causes an increased current in one of the sets of coils. This moves the armature and closes a local circuit, producing a click from the sounder. When both keys are depressed the currents in the two sets of coils of the relay connected with the line are neutralized as before, but the current flowing through the coils connected with the artificial line is greater and in the opposite direction, so it affects the relay and closes the circuit of the sounder. In addition to these methods there is also the bridge method, founded on the principle of the Wheatstone bridge. This method is used in the operation of the submarine cables. Duplex-working led to diplex, that is, two messages passing over a wire in the same direction at once, and this was followed by quadruplex and multiplex telegraphy. Quadruplex-working, whereby four messages were sent over one line, was the next development, and was first used by the Western Union Telegraph Company in 1874, and is now employed on all busy lines, particularly those between large cities. The first invention and one that has been most used was that of Edison, but improvement and new methods have been devised by Prescott, Gerrit Smith, and others.
One of the principal instruments used for the purpose of distinguishing different messages which are sent over a wire at the same time is the polarized relay, which has already been mentioned. The object of the instrument is to distinguish between messages sent with a positive current and those sent with a reversed current. Thus two such instruments can be placed in an office, connected to the same wire and adjusted so that one will respond only to the messages sent with a positive current, while the other will respond only to those sent with a negative current.
POLARIZED RELAY.
The way in which this is arranged will be seen from the illustrations. In the diagram n s is a hard steel permanent magnet, whose south end, s, has a slit in which the soft iron armature a is pivoted. To this armature a thin aluminum tongue, b, is attached, which by making contacts on either side completes either of two local circuits, and operates either of two sounders, according to which side the tongue is attracted. The coils on the magnet are so connected that a positive current attracts the tongue to one side, while a negative current attracts it to the other.
DIAGRAM SHOWING ACTION OF POLARIZED RELAY.
Automatic Transmission. The speed of the ordinary Morse instrument is limited to the rapidity with which the hand of the operator can move the key, and averages about 25 to 40 words a minute. To increase speed automatic apparatus is used which trebles, and in some cases quadruples the carrying capacity of a wire, securing at the same time mechanical accuracy in the relative size of the dots, dashes, spaces, etc. To effect this, three different instruments are required: First, there is a perforator, by which holes are punched in a paper slip to correspond with the signals required. The operator strikes three punches, the central one producing a central hole, which is of no avail electrically, only carrying forward the paper; the left-hand disk producing two holes, directly opposite to each other, on the sides of this central row, and that on the right producing two holes, placed diagonally to each other. The passage of the electric current is regulated by the position of the outer holes; those opposite each other admit of a momentary passage of the current through the second portion of the instrument, i.e., the ‘transmitter,’ which is used in sending the message, while the holes diagonally placed produce a lengthened mark corresponding to the dash. The following diagram represents the word ‘and,’ as shown on the punched slip:
PUNCTURED TAPE.
This would be printed
| • — | — • | — • • | ||
| a | n | d. |
The third portion of this instrument is the ‘receiver,’ in which the currents sent by the action of the punched slips in the transmitter are reproduced in the dots and dashes of the Morse code—the printing being, moreover, done with a mathematical accuracy which keying by hand cannot attain. The speed of transmission depends on the length of the line and state of the atmosphere; but the movements of the clockwork, both of the transmitter and receiver, are capable of adjustment to any speed below 120 words per minute.
A system of multiplex telegraphy invented by Delany and employed in Europe provides for the simultaneous transmission of a number of messages either in the same direction or part in one direction and the remainder in the opposite direction. The apparatus consists of a number of
