In European practise the main line is kept on open circuit. Polarized relays are used to work the local circuits. The currents for these have to be alternating in direction. When the line is not in use its ends are connected to earth at both ends, leaving the battery out of circuit.

Each intermediate station has its own main, or line battery for use when it desires to send a message. In the American system as first described, it will be seen that the main batteries are at most two in number.

For the details of the different apparatus, the following definitions may be consulted: Embosser, Telegraphic--Recorder, Morse--Relay--Relay Connection--Sounder.

514 STANDARD ELECTRICAL DICTIONARY.

Telegraph, Multiplex.

A system of telegraphy by which a number of messages can be transmitted in both directions over a single wire. The principles underlying the systems are the following:

Suppose that at the two extremities of a telegraph line two arms are kept in absolute synchronous rotation. Let the arms in their rotation, press upon as many conducting segments as there are to be transmissions over the line. A transmitting and receiving set of instruments may be connected to one segment at one end of the line, and another set to the corresponding segment at the distant station. For each pair of segments two sets can be thus connected. Then if the arm rotates so rapidly that the contacts succeed each other rapidly enough each pair of sets of instruments can be worked independently of the others. In practice this rapid succession is effected by having a number of contacts made for each pair during a single rotation of the arm or equivalent.

The multiplex system has been perfected by the use of La Cour's phonic wheel (see Phonic Wheel), and brought into a practical success by Patrick B. Delany, of New York.

Two phonic wheels rotate at each end of the line. They are kept in synchronous motion by two vibrating steel reeds of exactly the same fundamental note, and the axle of each wheel carries an arm whose end trails over the contacts or distributor segments already spoken of. The reeds are adjusted to vibrate at such speed that the trailer is in contact with each segment about 1/500 second. The number of groups of segments required for each working is determined by the r.e.t.a.r.dation of the signals owing to the static capacity of the line. To convert the rapidly recurring impulses of current into practically a single current, condensers are connected across the coils of the relay. One battery serves for all the arms.

Multiplex telegraphy can effect from two to six simultaneous transmissions over one wire. For two or four transmissions the method only distinguishes it from duplex or quadruplex telegraphy. The terms diode, triode, tetrode, pentode and hexode working are used to indicate respectively the simultaneous transmission of two, three, four, five, or six messages over one wire.

It will be seen that the multiplex process really a.s.signs to each transmission separate times, but divides these times into such short and quickly recurring intervals that the work is executed as if there was continuous contact. In no case is there the popular conception of the sending of several messages actually simultaneously over one wire. Each signal in reality has its own time a.s.signed it, divided into short periods of high frequency, and only utilizes the line when it is free.

515 STANDARD ELECTRICAL DICTIONARY.

Telegraph, Over-house.

An English term for telegraph lines led over houses and supported on standards on the roofs.

Telegraph Pole Brackets.

Arms for carrying insulators, which arms are attached to telegraph poles or other support. They vary in style; sometimes they are straight bars of wood gained into and bolted or spiked in place; sometimes they are of iron.

Telegraph, Printing.

Various telegraphs have been invented for printing in the ordinary alphabet the messages at the receiving end of the line.

Representative instruments of this cla.s.s are used for transmitting different market and stock reports to business offices from the exchanges. The type faces are carried on the periphery of a printing wheel, which is rotated like the hand of a dial telegraph, and against whose face a paper riband is pressed whenever the proper letter comes opposite to it. As each letter is printed the paper moves forward the s.p.a.ce of one letter. s.p.a.cing between words is also provided for. In the recent instruments two lines of letters are printed on the paper one above the other.

In England, and on the continent of Europe, printing instruments have received considerable use for ordinary telegraphic work. Hughes' type printer and Wheatstone's ABC telegraph meet with extensive use there for ordinary transmission.

Telegraph, Quadruplex.

Duplex telegraphy is the sending of two messages in opposite directions simultaneously through the same wire. Duplex telegraphy is the sending of two messages simultaneously in the same direction. The two combined const.i.tute quadruplex telegraphy. [SIC]

The system was suggested by Stark of Vienna and Bosscha of Leyden in 1855; the successful problem was solved by Edison in 1874.

The principle is based on the two orders of difference in electric currents; they may vary in strength or in direction. Thus we may have one instrument which works with change of strength of current only, the other with change of direction only. The two can be worked together if the direction of the current can be altered without alteration of strength, and if strength can be altered without alteration of direction. Double current and single current working are so combined that one relay works by one system of currents and another relay by the other system. A current is constantly maintained through the line. The relay operated by change in direction is a simple polarized relay which works by change of direction of current. The relay operated by change in strength is the ordinary unpolarized relay.

516 STANDARD ELECTRICAL DICTIONARY.

For the following description and the cuts ill.u.s.trating it we are indebted to Preece and Sivewright. The cut shows the arrangement of the apparatus and connections for terminal offices.

"Sufficient table room is provided to seat four clerks. The apparatus is arranged for the two senders to sit together in the centre, the messages to be forwarded being placed between them. The section on the left of the switch Q is known as the 'A' side, that on the right as the 'B' side of the apparatus.

K1 the reversing key, reverses the direction of the current. K2 is a simple key, known as the increment key; it is used simply to increase the strength of the current.

Fig. 320. QUADRUPLEX TELEGRAPH CONNECTIONS.

The way in which the keys K1 and K2 combine their action is shown by Fig. 321. E1 and E2 are the line batteries, the one having two and one-third (2-1/3) the number of cells of the other, so that if E1 be the electro-motive force of the smaller, that of the whole combined battery will be 3.3 E1. The negative pole of E1 is connected to z and z1 of K1 and the positive pole of E2 to a of K2 through a resistance coil s. A wire, called the 'tap' wire, connects the positive pole of E1 and the negative pole of E2 to b of K2. This wire has in it a resistance coil r2. The springs c and c1 of Kl are connected to the lever L of K2. Now, when both keys are at rest, the negative pole of E1 is to line through z, and the positive pole of E1 to earth through b of K2 and c of K1; the positive pole of E2 being insulated at a of K2.

517 STANDARD ELECTRICAL DICTIONARY.

There is thus a weak negative current flowing to line. When K1 alone is worked, the current of E1 is reversed. When K2 is worked alone, c of K1 is transferred from b to a, and the strength of the negative current going to line is increased through the increase of the electro-motive force from E1 to 3.3 E1 for the whole battery is brought into play. When K1 and K2 are depressed together, then the negative pole of E1 goes to earth through Z1; and the positive pole of E2 to line through a of K2 and c1 of K1 and a positive current, due to the whole electro-motive force 3.3 E1 goes to line. Hence the effect of working K1 is simply to reverse the current, whatever its strength, while that of K2 is to strengthen it, whatever its direction.

The resistance coil s, of 100? resistance, is called a spark coil, because it prevents the high electro-motive force of the whole battery from damaging the points of contact by sparking or forming an arc across when signals are sent; and the resistance r2 is made approximately equal to the combined resistance of E2 and the spark coil, so that the total resistance of the circuit may not be altered by the working of the apparatus.

Fig. 321. QUADRUPLEX TELEGRAPH.

A1 and B1 (Fig. 320) are the relays which are used to respond to the changes in the currents sent by the keys K1 and K2 at the distant station.

A, is a simple polarized relay wound differentially, each wire having a resistance of 200 [omega], and so connected up as to respond to the working of the reversing key K1 of the distant station. It acts independently of the strength of the current, and is therefore not affected by the working of the increment key K2. It is connected up so as to complete the local circuit of the sounder S1 and the local battery l1 and forms the receiving portion of the 'A' side.

B, is a non-polarized relay also wound differentially, each coil having a resistance of 200 [omega]. It responds only to an increase in the strength of the current, and therefore only to the working of the increment key K2 of the distant station.

[Transcriber's note: In current usage upper case omega indicates ohms and lower case omega denotes angular frequency, 2*PI*f.]

518 STANDARD ELECTRICAL DICTIONARY.

The relay spring is so adjusted that the armatures are not actuated by the weak current sent from E by the key K1.

In its normal position this relay completes the circuit of the local battery through the sounder S. This sounder S, called the uprighting sounder, acts as a relay to a second sounder, S2, called the reading sounder, which is worked by another local battery, l2. Of course, normally, the armature of S is held down and that of S2 is up, but when the tongue t moves, as it does when the increment key K2 is depressed so as to send the whole current to line, then the current from l is interrupted, and the circuit of l2 is completed by the rising of the armature of S, causing the reading sounder S2 to work. This is the 'B'

side.

R is a rheostat for balancing the resistance of the line, as used in duplex working.

C is a condenser used for compensating the static charge of the line. It is provided with an adjustable r.e.t.a.r.dation coil, R1, to prolong the effect of the compensating current from the condenser.

G is a differential galvanometer, used for testing, and for facilitating adjustment and balancing.