Q is a switch for putting the line to earth, either for balancing, or for any other purpose. There is on the earth wire leading from Q a resistance coil, r1, equalling approximately the resistance of the whole battery, 3.3 E1, and the resistance s.

The connections shown in Fig. 321, are for an 'up' office. At a 'down'

office it is necessary to reverse the wires on the two lower terminals of the galvanometer and the two battery wires on the reversing key K1.

The keys K1 and K2 are, for repeaters, replaced by transmitters.

The adjustment of this apparatus requires great care and great accuracy.

Its good working depends essentially on technical skill that can only be acquired by patience and perseverance.

Faults in working generally arise from careless adjustments, dirty contacts, loose connections, battery failures, and the ordinary line interruptions, but there are no troubles that are beyond the reach of ordinary skill, and it can be safely said that, within moderate distances, wherever and whenever duplex working is practicable, then quadruplex working is so too."

The above is a typical quadruplex bridge system. There is also a differential system, the full description of which, in addition to what has been given, is outside of the scope of this work.

519 STANDARD ELECTRICAL DICTIONARY.

Telegraph Repeater.

An extension of the relay system, adopted for long lines. A repeating station comprises in general terms duplicate repeating apparatus. One set is connected for messages in one direction, the other for messages in the opposite direction. The general operation of a repeating set is as follows. The signals as received actuate a relay which by its local circuit actuates a key, which in ordinary practise would be the sounder, but in the repeater its lever opens and closes a circuit comprising a battery and a further section of the line.

Repeaters are placed at intervals along the line. Each repeater repeats the signals received for the next section of line with a new battery. It represents an operator who would receive and repeat the message, except that it works automatically.

The Indo-European line from London to Teheran, 3,800 miles long, is worked directly without any hand retransmission, it being carried out by five repeaters. This gives an average of over 500 miles for each repeater.

[Transcriber's note: ? 650 miles for each repeater.]

Repeaters introduce r.e.t.a.r.dation, and each repeater involves a reduction in the rate of working. Yet in many cases they increase the speed of a line greatly, as its speed is about equal to that of its worst section, which may be far greater than that of the whole line in one.

Synonym--Translater.

Telegraph Signal.

In the telegraph alphabet, a dot, or dash; the signal or effect produced by one closing of the circuit. A dash is equal in length to three dots.

The s.p.a.ce between signals is equal to one dot; the s.p.a.ce between letters to three dots; and the s.p.a.ce between words to six dots.

Telegraph, Single Needle.

A telegraph system in which the code is transmitted by the movements of a needle shaped index which oscillates to right and left, the left hand deflection corresponding to dots, the right hand deflection to dashes.

The instruments for sending and receiving are combined into one. The needles are virtually the indexes of vertical galvanometers. In one form by a tapper key (see Tapper), in another form by a key worked by a drop-handle (the drop handle instrument), currents of opposite directions are sent down the line. These pa.s.s through both instruments, affecting both needles and causing them to swing to right or left, as the operator moves his key.

As galvanometer needle or actuating needle a soft iron needle is employed, which is polarized by the proximity of two permanent magnets.

This avoids danger of reversal of polarity from lightning, a trouble incident to the old system.

520 STANDARD ELECTRICAL DICTIONARY.

The cut, Fig. 322, shows a single needle telegraph instrument of the tapper form. The action of the tapper can be understood from the next cut.

Fig. 322. SINGLE NEEDLE TELEGRAPH INSTRUMENT, DOUBLE TAPPER FORM.

Fig. 323. DOUBLE TAPPER KEY FOR SINGLE NEEDLE TELEGRAPH.

C and Z are two strips of metal to which the positive and negative poles of the battery are respectively connected. E and L are two metallic springs; E is connected to earth, L is connected to the line; at rest both press against Z. If L is depressed so as to touch C, the current from the battery goes to the line by the key L, goes through the coils of the distant instrument and deflects the needle to one side, and then goes to the earth. If the key E is depressed, L retaining its normal position, the direction of the current is reversed, for the other pole of the battery is connected to the earth and the reverse current going through the coils of the distant instrument deflects the galvanometer needle to the other side.

In the drop-handle type an a.n.a.logous form of commutator worked by a single handle produces the same effects.

521 STANDARD ELECTRICAL DICTIONARY.

Telegraph, Wheatstone, A. B. C.

A magneto-electric telegraph of the dial system. An alternating current magneto-generator is turned by hand and by depressing keys its current is admitted to or cut off from the line and receiver's instrument. The message is received by a dial instrument working by the escapement motion described under Telegraph, Dial.

Telegraph, Writing.

A telegraph in which the message is received in written characters. The transmitter includes a stylus which is held in the hand and whose point bears against the upper end of a vertical rod. The rod is susceptible of oscillation in all directions, having at its base a spring support equivalent to a universal joint.

The stylus is moved about in the shape of letters. As it does this it throws a series of resistances in and out of the circuit.

At the receiving end of the line the instrument for recording the message includes two electro-magnets with their cores at right angles to each other and their faces near together at the point of the angle. An armature is supported between the faces and through it a vertical rod carried by a spring at its bottom rises. These magnets receive current proportional to the resistances cut in and out by the motions of the other rod at the transmitting end of the line. These resistances are arranged in two series at right angles to each other, one for each magnet. Thus the movements of the transmitting stylus and rod are repeated by the end of the rod in the receiving instrument. A species of pen is carried at the end of the rod of the receiving instrument, which marks the letters upon a riband of paper which is fed beneath it.

Telemanometer. Electric.

A pressure gauge with electric attachment for indicating or recording its indications at a distance.

It is applicable to steam boilers, so as to give the steam pressure in any desired place.

Telemeter, Electric.

An apparatus for electrically indicating or recording at a distance the indications of any instrument such as a pressure gauge, barometer or thermometer, or for similar work. The telemanometer applied to a boiler comes into this cla.s.s of instrument.

Telephotography.

The transmission of pictures by the electric current, the requisite changes in the current being effected by the action of light upon selenium. The picture is projected by a magic lantern. Its projection is traversed by a selenium resistance through which the current pa.s.ses.

This is moved systematically over its entire area, thus const.i.tuting the transmitter, and synchronously with the motion of the selenium a contact point at the other end of the line moves systematically over a sheet of chemically prepared paper. The paper, which may be saturated with a solution of pota.s.sium ferrocyanide and ammonium nitrate, is stained by the pa.s.sage of the current, and by the variation in intensity of staining, which variation is due to variations in the current, produced by the effects of the light upon the selenium, the picture is reproduced.

522 STANDARD ELECTRICAL DICTIONARY.

Telepherage.

An electric transportation system, hitherto only used for the carrying of ore, freight, etc. Its characteristic feature is that the electric conductors, suspended from poles, supply the way on which carriages provided with electric motors run. The motors take their current directly from the conductors.

There are two conducting lines, running parallel with each other, supported at the opposite ends of transverse brackets on a row of supporting poles. At each pole the lines cross over so that right line alternates with left, between consecutive pairs of poles.