39. A Telegraphic Circuit consists of one or more batteries, the line wire, the instruments and the earth. When the circuit is very short a return wire is frequently used instead of the earth.
40. Owing to the immense rapidity with which the electric force is propagated throughout a circuit, any effect which can be produced at hand can be produced in any other part of a circuit, however distant, at the same instant of time, subject to a diminution of force, arising from causes which diminish the quantity of electricity or the force of the current before its arrival at the distant end, thus weakening its effect. The principal causes of this diminution are the resistance of the circuit and defective insulation, in consequence of which a portion of the current escapes from the line to the earth, and returns without traversing the distant portion of the circuit.
41. The effective force of the current leaving the battery depends upon two things---the tension of the battery which sets the current in circulation, and the resistance the current encounters in traversing the circuit.
42. Resistance of the Circuit.--- This depends upon the length and size of the conductor, and the material of which it is composed. In an ordinary telegraphic line wire the resistance is in direct proportion to its length, and also in inverse proportion to its weight per mile. Thus, 150 miles of No. 8 wire will conduct as well as 100 miles of No. 10 wire, and as great an effect can be produced at its remote end with a battery of equal tension. There is, therefore, a great advantage in using the larger sizes of wire in the construction of lines intended to be worked in long circuits.
43. Electrical Measurement.--- In order to institute a comparison between the resistances of different circuits, etc., a standard has been fixed upon by the British Association, called the Ohm, which is equivalent to about 1/16 of a mile of galvanized No. 9 iron wire, such as is usually employed in the construction of telegraph lines. This standard unit of resistance is now made use of by the English electricians.
44. Resistance Coils.--- As no battery is constant in its power, and no magnet uniform in its strength, neither of these can be made use of as an accurate basis of comparison. Resistance coils, composed of wire of certain alloys of metals, carefully prepared, have been found not to vary 1/1,000,000 in eight years. The only variation is that due to difference in temperature, which may be readily calculated and allowed for when necessary.
It will, therefore, be understood that the ohm is a unit of resistance in the same manner that an inch is a unit of length, or a pound a unit of weight.
45. A Telegraphic Circuit, in its simplest form, is shown in fig. 9. A and B represent two stations. The circuit may be traced as follows: From the + pole of the battery E to the key K (52) and electro-magnet M, thence through the line L to the other station, electromagnet M' and key K' to the earth at G', and thence through the earth, as represented by the dotted line, to the -pole of the battery E. A continuous current will therefore flow through the circuit as long as it remains uninterrupted, and the armatures of the electro-magnets M and M' will be attracted by the cores, but if the circuit be broken by means of one of the keys, K or K', both electro-magnets will be demagnetized. Thus, the breaking of the circuit at either station affects the electro-magnets of both, as they are in the same circuit.
46. The Earth Circuit.--- In thus using the earth as part of the circuit, it is found that it offers, practically, no resistance to the passage of the current. Although comparatively a poor conductor, it is an infinitely large one in proportion to the wire, and, therefore, its resistance is not appreciable (42).
47. Arrangement of Batteries.--- In practice it is usual to divide the battery E into two parts, placing half at each end of the line, for reasons which will hereafter appear. It is important, however, when this is done, that the positive pole of one battery should be connected with the negative pole of the other, otherwise they would neutralize each other, and no effect would be obtained. In such a case the batteries are said to be reversed.
48. Intermediate Stations.--- It is evident that intermediate stations may be introduced at any point upon the line shown in the above figure, each being provided with an electro-magnet and key, forming part of the circuit, and that the breaking and closing of the circuit at any of these points will affect all the electro- magnets through which it passes, in the same manner and at the same instant of time.
Any desired number of intermediate stations may be placed upon a line until the combined resistance of their electro-magnets reduces the strength of the current below that required for the convenient working of the circuit.
49. The Morse System.--- The principle of the Morse system of telegraphy consists in conveying arbitrary signals my means of the magnetization and demagnetization of an electro-magnet, by the alternate breaking and closing of a voltaic circuit in the manner above explained. The conventional alphabet used in America for this purpose is given in another part of this work.
50. Other Telegraphic Systems.--- The type printing telegraph, employing the ``Combination'' instrument of Phelps, is the only system other than the Morse now in use upon the public lines in the United States. The limited extent to which it is employed renders it unnecessary to give a detailed description of its construction and mode of operation in a work of this kind. The electro- chemical telegraph of Bain, and the beautiful type-printing instruments of House and Hughes, were formerly extensively employed in this country. The former has now given place to the Morse, while the two latter have been superseded by the equally rapid and more simple and effective instrument of Phelps.
In addition to these, the magneto-electric dial instrument of Edmands & Hamblet, and the electro-magnetic alphabetical instrument of Chester are finding extensive employment upon private lines, where extreme rapidity of transmission is not required, thus rendering the employment of skilled operators in such cases unnecessary.