The instrument C C shown at A and P is an electrical condenser. It performs a very useful function in duplex and quadru plea telegraphy as follows. The artificial line coils R R which are used to (balance with are usually composed of spools of fine German silver wire, wound double, or non inductively, so that they may not possess any perceptible magnetic effect or inductance. These coils have no static capacity. The main line it is known does possess static capacity. Hence at the moment of charging and dis charging the line there is a momentary inrush and outrush of current into and from the line greater than that due to the ohmic resistance of the line. This would produce a momentary inequality in the current in the coils of the relays unless equal capacity were given to the artificial line. This is done by adding the condensers C to the artificial line. These condensers are adjustable and by rneans of metal plugs more or less capacity may be added until a static balance is obtained. The rheo stats are also adjustable in order that the re sistance balance may be readily obtained. • The Quaciruplex .(Edison).—The Stearns duplex depends for its operation upon the in crease or decrease of the strength of current on the line, regardless of the direction (polarity) of the current, whilst the polar duplex depends upon changes in the direction of the current re gardless of current strength. In the operation of the Edison quadruplex system both of these principles are combined on one wire; the in strwnents used being the transmitting key (transmitter) and Morse relay (the neutral relay) of the Stearns duplex, and the pole changer and polarized relay of the polar duplex.
The relays are wound differentially for the purpose stated previously and rheostats, con densers, etc., are employed, as in the duplex systeins described, In the Stearns duplex, when the transmitter is open there is no electro motive force to the hne, the wire in that case being placed directly to ground. Since the polar duplex depends for its operation upon the reversal of polarity, provision is made for this requirement In the quadruplex by so arranging the connections of the transmitter K that when the latter is gopeno a small por tion, 1, of battery B is left in the circuit to be operated by the pole-changer PC. When K is dosed, the full battery B is reversed by the pole-changer. On the other hand, when the small portion 1 of battery B is to the line, and only a weak current is traversing the line, the adjustment of the retractile spring is such that the arrnature a is withdrawn from the core; while, when the key K is closed, and the full strength of batterypasses to line, the arma ture is attracted. Both stations are similarly equipped. Thus an operator may manipulate key K and the consequent increase and decrease in the current strength will operate the distant neutral relay N. But the operation of key K
will not practically affect the distant polanzed relay P, inasmuch as that key does not alter the direction of current on the line. On the other hand the pole-c.hanger PC will operate the distant polanzed relay, but will not oper ate the distant neutral relay. Thus two mcs sages may be sent in opposite directions at once over a quadruplex circuit; this system, there fore, giving the eqtuvalent of four wires from dne. The three wires thus gained are termed °phantom') wires.
Many details of apparatus required in practice are, for lack of space, omitted here, but complete details of these systems are given in the author's
Submarine Cable Telegraphy.—The speed of signaling through an electrical conductor is inversely proportional to the product of its electrostatic capacity and the resistance. The capacity and resistance of a conductor increases directly with its length. The capacity also varies with the material comprising the in sulating medium (see Euscrarc CONDENSER). The high electrostatic capacity and the great length of submarine cables conduce to slow signaling. When a long cable is connected with a source of electromotive force the charge is communicated to the distant end almost im mediately., but the amount of charge at first arriving is extremely small and the charge or current rises slowly thereafter to its maximum. The discharge is approximately as gradual as the charge. H,ence a desideratum in submarine telegraphy is a receiving instrument that will respond to a very feeble current, for the wealcer the current required the shorter will be the time of charging and discharging the cable, and the more rapid will be the rate of signal ing. The first most successful receiving in strument devised for this work was the Thom son mirror galvanometer (see GALVANOMETER). The light from a lamp is thrown upon the mirror and the light is reflected back upon a screen. .The mirror is carried on a suspension system including several very small magnetic needles, which are in the centre of a coil of fine wire, the coil being in the circuit of the cable. Minute pulsations of current cause the needles to be deflected to the right or.left ac cording to the direction of the currents. This causes the spot of light on the screen to be de flected to the right or left. A deflection to the right constitutes a dash, one to the left a dot. The direction of the deflections irregulated by a special form of key (K, Fig. 15) at the sending end (a tapper) by means of which positive and negative currents may be trans mitted. The mirror receiver gives no record of the message, the signals being.written down by the operator as they are received.