In Fig. 3 is shown theoretically the essen tial apparatus of a Morse telegraph equip ment at two terminal stations, X, Y. B is the battery; K is the Morse key. S is an iron core or bar, arranged in the shape of a horse shoe, around which a coil of fine wire is wound. A is a small piece of iron, termed the (armature,* which is mounted on a pivoted lever r. A retractile spring x with draws the armature from the ends of the core S when the latter is demagnetized. The play of the lever is limited by the stops c c'; the front one of which c, is a contact point that acts as a key for a local circuit in which are included small battery B' and the coils of the sounder s'. The relay R consists essentially of the iron core, S, the coil of wire and the lever r. It also has devices for adjusting the • position of the armature relative to the ends of the core, and screw posts for facilitating the connections of the wires leading into and out of the instrument. (See Fig. 5). .
The object in using the sounder s' in Morse telegraphy is to obtain louder sound than is practicable with the Morse relay. The magnetism developed in a given electromagnet, such as.a relay or sounder, increases with the strength of current, also with the number of convolutions of wire in the coils, within certain limits. In fact, up to the point of magnetic saturation of the iron, the resulting magnetism is directly proportional to the product of the current strength in amperes by the number of convolutions, or turns, of wire; which product is termed the "ampere turns." Saturation is, hori•ever, seldom or never reached in the mag nets used in telegraphy. Again, with a given electromotive force current strength is inversely proportional to the resistance of the circuit, and as the resistance of a wire of given diameter increases directly with its length, the longer the wire, the weaker will be the current. It is found that to produce the clear, loud click of the ordinary Morse sounder, about 25 am pere is required, while a main line relay will operate freely with .040 ampere (40 milliam peres). To operate a sounder placed in a main line having 1,200 ohms resistance would require an electromotive force of about 300 volts. It is, therefore, more economical and in other ways better to employ a lower electromotive force, about 60 volts in this case, and a relay having many turns of finer wire than • the sounder (about No. 30 B & S gauge) and a light arma ture, not designed to produce a large volume of sound, which relay by means of its armature lever and a ((logical" battery is then caused to operate a sounder as indicated in Fig. 1.
The sounder is constructed on the same prin ciples as the relay, but the wire with which it is wound is larger, about No. 24 B & S gauge, and its armature and lever are heavier. Since the lever r of the relay controls the local cir cuit of the sounder it follows that as the main line is closed and opened, thereby closing and opening the relay, the local circuit will be similarly closed and opened, by which actions the electromagnet of the sounder will be cor respondingly alternately magnetized and de magnetized, and the armature lever will be at tracted by the magnetism and withdrawn by its retractile spring. The motions of the sounder thus produced cause the long and short sounds which are translated into letters of the alphabet by an expert operator. When a Morse register or ink recorder is used in place of the sounder these dots and dashes are embossed or printed on strips of paper from which the message may then le trans cribed by any one familiar with the dot and ash code. Plainly many more relays may be placed in the main circuit than are shown in Fig: 3. It is not uncommon in this country to have 30 or 40 relays in one Morse telegraph line, and with but two main batteries, one at each end. When any one of the keys of a Morse telegraph circuit is open none of the remaining keys can close the circuit, and when any one key is operated all the relays on the same wire will be operated concurrently, by reason of the opening and closing of the circuit by that key. Any attempt to operate two keys at once on this circuit results in the clashing of signals. Such circuits are termed °single" or "simplex," inasmuch as but one message at a time can be sent over them. This fact distinguishes simplex from multiplex cir cuits over which two or more messages can be transmitted simultaneously, instances of which will be given herein.
The arrangement of Morse circuits just described is termed the "closed circuit' method of operation, from the fact that the circuit is normally closed with current on the line. In Europe the Morse circuits have generally been operated on what is termed the ((open circuit' plan, which consists in so arranging the appa ratus that the battery shall only be placed to the line when a message is to be transmitted; at other times the battery is disconnected from the line. For this purpose a key with front and back contacts is used, similar to K in Fig. 12; battery being placed as there shown, and the receiving instrument being placed in the circuit of the back contact at x in that figure. In the open circuit method battery is necessary at every station.