TIIE RELAY. There are several methods by which the transmission of signals is facilitated or accelerated. First among these may be placed • the relay. In the previous description of the Morse sysiern, we have assumed the instrument to be worked directly by the current sent along the line. On long circuits, however, direct work ing could only be accomplished by great battery power. owing to inevitable loss by leakage, ex perienced by a current before it reaches its destination. Consequently what is known as a relay is used and the sounder or recording in strument is worked by a local current' derived from a focal battery at the receiving station. This is accomplished in the following way: The receiving instrument connected to the line in stead of being made heavy enough to give an audible sound by which the signal may he under stood, is made extremely light and delicate so that it can be operated by a weak current.
This relay consists of an electromagnet wound with a large number of turns of insulated wire and an armature on a lever to which is attached a contact point which strikes another stationary contact when the lever is drawn toward the mag net, closing a circuit through these extra points in precisely the same way that the usual signal ing key is worked. These contacts are connected by short wires with a regular sounder and a small battery. Whenever the lever of the relay is attracted it closes this extra or local circuit and causes the sounder to respond to every signal precisely as if it were connected directly to the main line.
DITPLEx-Womuxu. The first suggestion of using a single line for more than one message was due to Moses G. Farmer of Boston, who in 1852 devised a method for this purpose and car ried on successful experiments on a small scale. In 1353 Dr. Wilhelm Gintl first made use of the differential relay, and on the line between Vienna and Prague worked a method from which the modern practice of duplex telegraphy may be said to date. This system was rendered more successful in the following year by Carl Fisehen of Hanover, and his invention was acquired" by Siemens and Halske of Berlin. who had almost simultaneously devised a similar method. This improved system was introduced into Austria and from time to time there followed improvements devised by Stark and others. In England a du plex system was invented by Preece, while in other countries various methods from different inventors were forthcoming. In 1868 J. B. Stearns of Boston invented a system of duplex telegraphy which has since been used most suc cessfully on many lines throughout the United States. The theory and practice of modern du plex telegraphy are somewhat complex and diffi cult to understand, but an outline of the prin ciples involved may be given. In the differential method, which is largely used on land lines, there is what is known as a differential relay, that is a relay wound with two sets of coils in each of which the current flows in a different direction. Consequently when two currents of equal inten sity pass through such a relay their effects are neutralized and the wire does not become mag netized. At each station a relay of this kind is provided, and there are two complete circuits, one of which includes the line wire, while the other consists of resistance coils equivalent in resistance to the line and known as the artificial line. The key and battery are common to both circuits, the points of divergence being at the re lay and at the ground. if the key at one
station is depressed the current will flow through both sets of coils of the relay at this station without producing any magnetizing effect, hut at the distant station it will traverse but one set of coils and will accordingly cause the relay to operate the local sounder. The same thing of course holds good for the distant station. If both keys are depressed simultaneously the two batteries will combine to produce an increased strength of current flowing through one set of the differential coils of each relay, and by virtue of this preponderance of current the armature of the relay will be magnetized. A modification of this method is known as the polar duplex sys tem and employs at each station two batteries or dynamos, so arranged that the direction of the current flowing in the line depends on whether the key is at rest or depressed. The current divides as before at the relay, which, instead of being wound differentially, is known as a poor relay. Normally the currents going to the line flow from each station in such directions that they neu tralize each other, while the other portions of the divided circuit flow over an artificial line and into the ground. Now when a signal is to be sent, the operator by means of a key or pole changing instrument reverses the direction of the current, which passes through his relay without effect, but disturbs the conditions in that instru ment at the distant station and causes an in creased current in one of the sets of coils. This moves the armature and closes a local circuit, producing a click from the sounder. When both keys are depressed the currents in the two sets of coils of the relay connected with the line are neutralized as before, but the current flowing through the coils connected with the artificial line is greater and in the opposite direction, so it affects the relay and closes the circuit of the sounder. In addition to those methods there is also the bridge method, founded on the principle of the Wheatstone bridge. This method is used in the operation of the submarine cables. Du plex-working led to diplcx, that is, two messages passing over a wire in the same direction at once, and this was followed by quadruples and multi plex telegraphy. Quadruplex-working, whereby four messages were sent over one line, was the next development, and was first used by the West ern Telegraph Company in 1874, and is now employed on all busy lines, particularly those between large cities. The first invention and one that has been most used was that of Edison, but improvement and new methods have been devised by Prescott, Gerrit Smith, and others.
One of the principal instruments used for the purpose of distinguishing different. messages which are sent over a wire at the same time is the polarized relay, which has already been men tioned. The object of the instrument is to dis tinguish between messages sent with a positive current and those sent with a reversed current. Thus two such instruments can be placed in an office, connected to the same wire and adjusted so that one will respond only to the messages sent with a positive current, while the other will re spond only to those sent with a negative current.
two local circuits, and operates either of two sounders, according to which side the tongue is attracted. The coils on the magnet are so con nected that a positive current attracts the tongue to one side, while a negative current attracts it to the other.