The Standard Electrical Dictionary Part 152

Fig. 314. DUPLEX TELEGRAPH, DIFFERENTIAL SYSTEM

Suppose that B is sending a dash, and it begins while A's key is raised.

The line and compensation currents in B's receiving instrument neutralize each other and no effect is produced, while A's receiving instrument begins to register or indicate a dash. Now suppose A starts to send a dash while B's is half over. He depresses his key. This sends the two opposite currents through his magnet. His line current neutralizes B's working current so that the compensation currents in both receiving instruments hold the armatures attracted for the two dashes. Meanwhile A's dash ends and he releases his key. At once his line current ceases to neutralize B's line current, his receiving instrument is actuated now by B's line current, while B's receiving instrument ceases to be actuated by the compensation current.

Two a.s.sumptions are made in the above description. The line currents are a.s.sumed to be equal in strength and opposite in direction at each station. Neither of these is necessary. The line current received at a station is always weaker than the outgoing line current, and it is the preponderance of the compensation current over the partly neutralized line current that does the work. As this preponderance is very nearly equal to the line current received from the distant station, the signals are actuated by almost the same current, whether it is compensation or line current.

509 STANDARD ELECTRICAL DICTIONARY.

Both line currents may coincide in direction. Then when the two keys are depressed, a line current of double strength goes through both receiving instruments and both work by preponderance of the double line current over the compensation current. In other respects the operation is the same as before described.

Fig. 315. DUPLEX TELEGRAPH, DIFFERENTIAL SYSTEM.

Fig. 316. DIFFERENTIAL DUPLEX TELEGRAPH CONNECTIONS.

The cut shows a diagram of the operation of one end of the line. R and R are resistances, E and E are earth contacts, and the two circles show the magnet of the receiving instrument wound with two coils in opposition. The battery and key are also shown. It also ill.u.s.trates what happens if the key of the receiver is in the intermediate position breaking contact at both 1 and 2. The sender's line current then goes through both coils of the receiving instrument magnet, but this time in series, and in coincident direction. This actuates the instrument as before. Owing to the resistance only half the normal current pa.s.ses, but this half goes through twice as many coils or turns as if the receiver's key was in either of the other two positions.

In actual practice there are many refinements. To compensate for the varying resistance of the line a rheostat or resistance with sliding connection arm is connected in the compensation circuit so that the resistance can be instantly changed. As the electro-static capacity of the line varies sectional condensers are also connected in the compensation circuits.

510 STANDARD ELECTRICAL DICTIONARY.

Telegraph, Facsimile.

A telegraph for transmitting facsimiles of drawing or writing. The methods employed involve the synchronous rotation of two metallic cylinders, one at the transmitting end, the other at the receiving end.

On the transmitter the design is drawn with non-conducting ink. A tracer presses upon the surface of each cylinder and a circuit is completed through the two contacts. In operation a sheet of chemically prepared paper is placed over the surface of the receiving cylinder. The two cylinders are rotated in exact synchronism and the tracers are traversed longitudinally as the cylinders rotate. Thus a number of makes and breaks are produced by the transmitting cylinder, and on the receiving cylinder the chemicals in the paper are decomposed, producing marks on the paper exactly corresponding to those on the transmitting cylinder.

Synonyms--Autographic Telegraph--Pantelegraphy.

Telegraph, Harmonic Multiplex.

A telegraph utilizing sympathetic vibration for the transmission of several messages at once over the same line. It is the invention of Elisha Gray. The transmitting instrument comprises a series of vibrating reeds or tuning-forks, each one of a different note, kept in vibration each by its own electro-magnet. Each fork is in its own circuit, and all unite with the main line so as to send over it a make and break current containing as many notes superimposed as there are tuning forks. At the other end of the line there are corresponding tuning forks, each with its own magnet. Each fork at this end picks up its own note from the makes and breaks on the main line, by the principle of sympathetic vibration.

To each pair of operators a pair of forks of identical notes are a.s.signed. As many messages can be transmitted simultaneously as there are pairs of forks or reeds.

The movements of a telegraph key in circuit with one of the transmitting reeds sends signals of the Morse alphabet, which are picked out by the tuning fork of identical note at the other end of the line.

511 STANDARD ELECTRICAL DICTIONARY.

Telegraph, Hughes'.

A printing telegraph in very extensive use in continental Europe. Its general features are as follows:

The instruments at each end of the line are identical. Each includes a keyboard like a piano manual, with a key for each letter or character.

On each machine is a type wheel, which has the characters engraved in relief upon its face. With the wheel a "chariot" as it is termed also rotates. The type wheels at both stations are synchronized. When a key is depressed, a pin is thrown up which arrests the chariot, and sends a current to the distant station. This current causes a riband of paper to be pressed up against the face of both type wheels so as to receive the imprint of the character corresponding to the key. The faces of the wheels are inked by an inking roller.

Fig. 317. ELECTRO-MAGNET OF HUGHES' PRINTING TELEGRAPH.

The most characteristic feature is the fact that the current sent by depressing a key does not attract an armature, but releases one, which is then pulled back by a spring. The armature is restored to its position by the mechanical operation of the instrument. The magnet used is a polarized electro-magnet. Coils are carried on the ends of a strong powerful magnet. The coils are so connected that a current sent through them by depressing a key is in opposition to the magnetism of the permanent magnet so that it tends to release the armature, and in practice does so. This release permits the printing mechanism to act.

The latter is driven by a descending weight, so that very slight electric currents can actuate the instruments.

Synonym--Hughes' Type Printer.

Telegraphic Code.

(a) The telegraphic alphabet, as of the Morse System. (See Alphabet, Telegraphic.)

(b) A code for use in transmitting messages either secretly, or comprising several words or short sentences in one word, in order to economize in transmission. Such codes are extensively used in commercial cable messages.

512 STANDARD ELECTRICAL DICTIONARY.

Telegraph, Magneto-electric.

A telegraph in which the current is produced by magneto-electric generators. It has been applied to a considerable extent in England. The Wheatstone ABC or dial telegraph is operated by a magneto-generator turned by hand.

In this country the magneto-electric generator by which the calling bell of a telephone is rung is an example. The magneto-electric key (See Key, Magneto-electric) is for use in one kind of magneto-electric telegraphing.

Telegraph, Morse.

A telegraph, characterized by the use of a relay, working a local circuit, which circuit contains a sounder, or recorder for giving dot and dash signals const.i.tuting the Morse alphabet. The signals are sent by a telegraph key, which when depressed closes the circuit, and when released opens it. The two underlying conceptions of the Morse Telegraph system are the use of the dot and dash alphabet, and the use of the local circuit, which circuit includes a receiving instrument, and is worked by a relay, actuating a local battery. It would be difficult to indicate any invention in telegraphy which has had such far-reaching consequences as the one known as the Morse telegraph.

In other places the princ.i.p.al apparatus of the system will be found described. The cut Fig. 318, repeated here gives the general disposition of a Morse system. (See Circuit, Local.)

Fig. 318. DIAGRAM OF MORSE SYSTEM.

513 STANDARD ELECTRICAL DICTIONARY.

The key by which the messages are transmitted is shown in Fig. 319. M is a base plate of bra.s.s. A is a bra.s.s lever, mounted on an arbor G carried between adjustable set screws D. C is the anvil where contact is made by depressing the key by the finger piece B of ebonite. E, Fl are adjusting screws for regulating the vertical play of the lever. H is the switch for opening or closing the circuit. It is opened for transmission, and closed for receiving. By screws, L L, with wing nuts, K K, the whole is screwed down to a table.

Fig. 319. MORSE TELEGRAPH KEY.

In the United States the simplest disposition of apparatus is generally used. The main line is kept on closed circuit. In it may be included a large number of relays at stations all along the line, each with its own local circuit. There may be fifty of such stations. Battery is generally placed at each end of the line. Very generally gravity batteries are used, although dynamos now tend to supplant them in important stations.

As relays the ordinary relay is used. Its local circuit includes a sounder and local battery. The latter is very generally of the gravity type, but oxide of copper batteries (See Battery, Oxide of Copper) are now being introduced. At main or central offices, the terminals of the lines reach switch boards, where by spring-jacks and plugs, any desired circuits can be looped into the main circuit in series therewith.