The Siemens Arnzature,a device well known to electrical engineers, is of peculiar construction (fig. 2S). It is channelled longitudinally with two deep, broad grooves on opposite sides, within which is wound, also longi tudinally, a coil of fine insulated wire. The armature is supported on pivots above and below by the brass caps, F, F, and is rotated by means of the pill ion on its upper extremity, which gears into a toothed wheel turned by the handle, H. The numerical relation between the teeth of the wheel and pinion is such that one revolution of the wheel causes the pinion to revolve thirteen times, reversing the mag,netisin along the whole length on each side of the armature, and thereby inducing; in the coil, when the circuit is closed, thirteen positive and thirteen negative electrical pulsations of equal and opposite magnetic effect. When the coil is turned half round on its vertical axis, the polarity of the armature is also changed and a magneto-electric current induced in the armature coil in one direction, and on turning it half a revolution farther round, so that it takes up its former position, a current in the opposite direction is induced. The ends of the coil of wire which are attached to the brass caps are connected, one (s') to the earth-plate, and the other (s) to the coils of the electro-magnet of the indicator, and thence to the line. The pointer of the indicator at the receiving- station repeats the messag-e sent from the transmitting station letter by letter. Siemens has lately replaced the permanent steel magnets by electro-magnets. The residual mag-netisin in the electro-magnets is found to be sufficient to produce a current in the armature bv induction. This induced current of the armature in turn excites the electro-magnets, these in turn induce a still stronger current in the armature, and so on until the point of magnetic saturation is reached.
Circuits.—For the transmission of signals in the case of telegraphs op erated with batteries, t WO arrangements of the line may be employed, which are known respectively as the "open-circuit" and the "closed-circuit" plan. In the open-circuit plan the batteries at both ends of the line are open when the line is not in use, but are both closed whenever the key of either station is depressed. Thus the instruments at both stations are op erated when either one or both keys are depressed. In the closed-circuit plan, the circuit is closed and the current flows through the line when the latter is not in use. The transmission of signals, however, by this plan may be effected in two ways. In the Morse system, as operated in the United States, the keys at each station are provided with what is known as a "circuit-closer," and the connection remains unbroken so lon,g as neither station is transmitting. To send a message, the operator opens a switch of his key, thus interrupting the flow of the current. By operating his key the characters of the Morse alphabet are received at the other sta tion. 13y this plan of arranging the instruments of a line the transmission is effected by closing the circuit through the key. In a modified form of the closed-circuit system in use in Germany upon the railway-lines, the signals are directly transmitted by breaking instead of closing- the circuit.
Thc Keys used to make and break the circuit are shown in Figures 32 and 33 (pi. 57). Figure 32 represents a key for an open-circuit line in
which, when the line is not in use, a contact is maintained between c and c', through which a current may be received or may pass through some other station. By depressing the lever, however, the local battery is closed throug-h the contact, b. The closed-circuit key (fig. 33) is provided only with the contacts aloud n, through which the circuit is closed when the line is not used for transmission. The circuit is broken by depressing the lever. The Morse key (pi. 5S, _fig. 6), as employed in the United States with the circuit-closer or switch, is usually open. When the switch is open, the circuit is completed through the key by pressing upon the button of the lever, by which a platinum stud on its under side is brought in contact with a similar stud in the centre of an anvil. When the pressure of the finger is removed, a spring beneath the lever throws it up to its normal position, thus breaking the circuit.
The Plug Commutator is extensively used on lines operated by Morse instruments, with some modifications. Figure 4o (pi. 57) shows the plug commutator of Siemens ancl.Halske. It consists of three heavy pieces of brass separated from one another upon the insulated base and fur nished with holes, D, C, 1, and 2. LI and 1,2 are the line wires from adja cent stations. T is the key; R, the relay; S, the registering- instrument; the line battery; OB, the local battery; and G, the galvanometer which indicates the existence of a current in the line. The switch itself is dis ting-uished by the letter U. By inserting a metallic plug in hole D the station is cut out and Li and L2 are connected. The current then passes from Li over U (D) to L2. If the plug be inserted in C, the station will receive any message sent through. In this case the current from a station on the left will pass over LI, G, Ti, 2, R, U, L2, etc. If now the key T1 be depressed, the station is placed in working communication with all stations on the line. In this case the current passes in the direction Li, G, T1, 3, Z, (LB), K, L2. In like manner the holes and 2 serve for the insertion of the plug when the station wishes to communicate with the way station on the right or left respectively, or to receive communications from these way stations.
Te/egraph A/arinc.—A telegraph station is not complete unless provided with alarms (pl. 57, Jigs. 25, 27). Figure 41 exhibits a call-bell in which the armature, c, carrying the hammer, /e, is drawn back by the spring at c, when the current ceases to flow round the coils of the electro-magnet, AI, and is pressed ag-ainst the contact-spring, r, when the current is re-estab lished, by which the armature will be attracted, and the bell struck by the hammer. By this artifice a rapid making- and breaking of the circuit is effected, and a corresponding intermittent sounding of the bell. When a system of this kind is used as an annunciator, for example, in a hotel, the rooms from which the circuit must be closed and broken must be furnished with a push-button (fig. 42). Communication is established by pressing upon the button. In the bell-signals for railway use the current is utilized merely to release a train of clockwork, which by simple mechanical means rings the bell. Siemens and Halske's railway sig-nal-bell operating on this principle is shown in Figure 43.