Page:The New International Encyclopædia 1st ed. v. 19.djvu/118

TELEGRAPH. illustration, and consists of an electro-magnet of horseshoe form firmly mounted on a base. The coils are wound with insulated wire and are inclosed in cylinders of hard rubber or vulcanite. Supported by an upright casting of brass is a lever balanced on the points of two screws, which carries transversely to its length an armature of soft iron. This piece of iron is directly above the poles of the electro-magnet, so that when the current passes through the coils it is attracted with sufficient force to produce a sharp click when a screw at its end strikes the surface of the metal. By means of a spring, whose tension can be adjusted, the lever is held in such a position that the armature does not come in contact with the poles of the electro-magnet until the latter becomes magnetized by the passage of the current and attracts it. When the current ceases to flow and the core is demagnetized, the spring brings the lever back to its former position and another click results as the lever strikes a screw in the frame. If the sounder is to be used on a local circuit, as will be explained below, its resistance is generally about four ohms, but when it is on a main line where there are no relays it has a resistance of about twenty ohms, though these resistances are varied to suit conditions of line and battery or dynamo. In order to obtain a clear, loud click from the sounder it is necessary to have about one-quarter of an ampere of current flowing through the coils.



The ‘key’ by which signals are transmitted from the operator at the sending office is shown below in its usual form. It consists of a lever pivoted near the middle and carrying at one end a knob by which it is manipulated by the operator in sending signals. Two platinum contacts are fastened respectively to the under side of the lever and to an insulated piece on the upper side of the base. These contacts are exactly opposite to each other, so that they come together every time the key is depressed. The lever is held up, when the key is not depressed by the operator, by a small spiral spring. One end of the line wire is connected to the sounder and then to the metallic base of

the key from which the current passes through the pivots to the lever and the small contact point attached to its under side. One terminal of the battery is connected with the small insulated piece supporting the lower contact while the other goes to the ground or return wire. By this arrangement no current can pass through the line until the key or lever is depressed, since the terminal of the wire which is fastened to the contact is insulated and the current has no means of reaching the other part of the wire. When it is desired to send the current the knob is depressed, and the current from the battery, finding a passage between the two contacts, passes from one end of the wire to the other and proceeds upon the line wire to the distant station. The length of time during which the key is depressed determines the length of time between the upward and downward clicks of the sounder at the other end of the line. Thus if the operator presses down the handle and releases it quickly the keeper of the sounder will go down and up, immediately indicating a ‘dot’ or the letter ‘E,’ while if he presses the key and holds it down a moment before releasing it. there will be a longer interval between the downward and upper clicks of the sounder, and a ‘dash’ or the letter ‘T’ will be produced. In early instruments, before the operators read the messages by the clicks, the receiving instrument was arranged to make a record of the signals upon a moving strip of paper. For this purpose a pen or sharp point was attached to a lever, and a strip of paper, kept in motion by clockwork, was arranged to pass under the pen, so that a mark was produced by the pen every time the lever was pulled down by the magnet. If the lever was immediately released a dot was recorded, but if the lever was held down a moment a longer mark called a dash was produced.



The signals used at first were arranged for making a record with a crude instrument of this kind; and while the increasing skill of the operators soon enabled them to read the signals directly from the clicking of the instrument without looking at the tape, the signals are still spoken of as consisting of so many dots and dashes, as if recorded on paper. These registers are to-day but little used, being seen occasionally in district telegraph stations or with fire-alarm apparatus.

Before going further the details of the alphabet may be given. The only elements which can be combined for making telegraphic signals in the Morse system are the dot, the dash, and the space, and the alphabet is made up of the simplest combinations of these that can be made. The preference of the shortest combinations is given to the letters which are most frequently used. This alphabet or code of signals was due to Morse's assistant and collaborator, Alfred Vail, and was based on the frequency with which the different letters occur. This was verified by consulting with printers and examining their type cases. The simplest signal, a dot, was given to ‘E’ and the next simplest, a dash, to ‘T,’ those letters occurring most frequently in our language. Between the separate signals of every single letter comes the space which is equivalent to a dot except in the case of C, O, R, Y, Z, when a lengthened space equivalent to two dots is used.