Bright's BelL—A modificaticrn of the nee dle telegraph is that known as 4Bright's,Bell,p ie which two bells Of diff'--•eatt tene are struck by a hammer, one tone representing, for in stance, a dot; the other, a dash. In other instances, the needle of the needle system is caused to strike metal tubes of different tones, one tone indicating a deflection in one direc tion, the other tone a deflection in the opposite direction. These have been termed acoustic telegraphs.
Electric Chemical Telegraphs.— See Auto matic Telegraph Systems in this article.
Morse Electro-Magnetic Telegraph.— In 1824 Sturgeon of England discovered that when a current of electricity is caused to flow in an insulated coil of wire, surrounding a bar of soft iron (that is, well-annealed iron), the bar takes on magnetic properties, and when the cur rent ceases to flow in the coil the iron at once loses its magnetism. Employing these electro magnetic phenomena, Morse, in 1837, invented the telegraph system which bears his name, and which to-day is in extensive use in one form or another in every part of the world. It was then known that when a piece of soft iron is placed near a magnet there is a mutual mag netic attraction which tends to draw them to wther. This fact is also availed of in the Morse telegraph system as will be shown in the case of the Morse relay and sounder.
To obtain the flow of current in the coil of wire required to magnetize the iron bar, the circuit must be complete, and a source of electromotive force must be provided.
An electric acircuitp may be represented, as in Fig. 1, by a line or wire, W; battery, B, the thin and thick lines of which in the figure con ventionally represent the positive and negative plates or elements of a primary chemical bat tery. The source of electromotive force in telegraphy for many years in this country was the (bluestone* or (gravity)) battery, the ele ments of which are zinc and copper in a solu tion of sulphate of copper in a glass jar. These cells are arranged in series; two or three for local circuits and from 25 to 350 for main line batteries. Of recent years these batteries and the bichromate primary batteries employed in Europe, have given way largely to storage bat teries and dynamo machines as a source of elec tromotive force.
When dynamos are employed they are in some cases designed to develop about 70 volts each. Several of these machines are connected in series so that by tapping the machines at different points an electromotive force of 70, 140, 210 volts, etc., may be obtained. In other cases dynamos developing 70, 140, 300 volts, respectively, are employed. Two sets of such machines are provided, one set to furnish posi tive polarity, the other negative polarity, inas much as it is essential in the practice of teleg raphy to utilize currents of opposite direction or polarity; instances of which will be noted in connection with the descriptions of duplex and quadruplex telegraphy in this article.
Reverting to Fig. 1, G is a galvanometer, an instrument to indicate the presence of an electric current in a circuit, and which is con structed on the principle of the indicating needle of needle telegraph systems previously mentioned. The direction of the current in the circuit is assumed to be from the positive pole of the battery B to the galvanometer G, and back to the negative pole of the battery. The key K affords a ready means of (opening* and °closing,* or of (making" and (breaking* the circuit. When the key is closed, or down, the circuit is complete and current flows; when the key is (open* or up, the circuit is open or broken, and no current flows therein. An ordinary telegraph circuit without instruments is indicated in Fig. 2. This is termed a ground return circuit. A circuit like that indicated in Fig. 1 is termed a °round* or metallic circuit. In the case of a ground return circuit the earth acts virtually as though it were a wire of inappreciable electrical resistance, com pleting the circuit from end to end.
In the Morse telegraph system signals are transmitted electrically from one station to another by the opening and closing of the circuit, or wire, by means of the key, for short and long intervals, which acts operate the Morse relay and sounder or register, and cause them to produce sounds or printed characters corresponding to dots and dashes of what is known as the Morse telegraph alphabet.