CHAMBERS'S INFORMATION FOR THE PEOPLE. 



through two miles of wire. But the appeal to so 

 fickle an agent was fruitless. It was only when 

 the properties of the galvanic current became 

 known, that a solution was found to the problem. 



The essential parts of every electric telegraph 

 are three. There must be, first, a complete circuit 

 for the current to traverse, including one or 

 more batteries for the generation of the current. 

 Secondly, there must be a communicator, or 

 means of producing the signals at the sending 

 station. Thirdly, there must be an indicator, or 

 apparatus for shewing the signals at the receiving 

 station. 



With regard to the first, there is not much 

 variety or choice of contrivance. A single insu- 

 lated wire is enough to put two stations in tele- 

 graphic communication. Although two wires were 

 at first supposed to be necessary, it was soon 

 found that the earth might be made to act the 

 part of a return wire to complete the circuit. 

 Thus the current passes from one pole of the 

 battery through the communicator, along the insu- 

 lated wire to the distant station. After producing 

 the desired signals there, it passes to a large 

 metal plate buried deep in the earth. A similar 

 plate is sunk at the signalling station, and is con- 

 nected with the other pole of the battery there. 

 Now, we may either regard the earth between the 

 plates as a huge conductor, allowing the currents 

 from the opposite poles to neutralise that is, as 

 a real return path or we may simply suppose 

 that equal amounts of positive and negative elec- 

 tricities are drawn off at each end. The current 

 depends on a constant neutralisation of the 

 opposite electricities polarised at the opposite 

 ends of the battery ; and it matters not 

 whether they neutralise directly, or the earth 

 neutralise or drain off both together. In 

 order to insure a proper draining off at each 

 end, a very good earth connection, or ' earth,' 

 must be secured, as in the case of a lightning- 

 conductor. One good 'earth' serves for all 

 the circuits of a telegraphic station. 



There are three forms of insulation neces- 

 sary, according as the wire is carried above 

 ground, or under ground, or under water ; and 

 the three forms of line are named aerial, 

 subterranean, or submarine, accordingly. ^ 



For aerial telegraphs, the wire is usually 

 of galvanised iron, and is attached to glass 

 or porcelain insulators, which are fixed to wooden 

 posts some twenty feet high. Much attention has 

 been paid to the manufacture of good insulators, 

 as everything depends on proper insulation. A 

 small loss or leakage at one pole may be incon- 

 siderable; but in a long line, where thousands 

 have to be taken into account, the loss may be 

 so great, that only a small part of the current that 

 sets out from the battery ever reaches the distant 

 station. 



Underground lines must have the insulation 

 perfectly continuous, and the wire is, for that 

 purpose, covered with gutta-percha or india-rub- 

 ber. These lines are not much used, on account 

 of the difficulty of 'faultless' insulation, the 

 deterioration of the percha or rubber, and the ex- 

 pense of repairs. 



In the case of submarine lines or cables, the 

 very greatest care must be taken in the insulation. 

 But on account of their importance, they will be 

 specially described under Submarine Telegraphy. 

 278 



The communicator assumes different forms 

 according to the plan of the indicator or recording 

 instrument But the general principle is in every 

 case the same. It is merely an apparatus for 

 rapidly interrupting the circuit, and completing it 

 again, or, in some cases, for rapidly changing the 

 pole of a battery with which the wire communi- 

 cates. 



The indicator is perhaps the most interesting 

 part of the telegraph, and the number of inven- 

 tions connected with it is almost endless. In 

 some, the signals are given by the deflection of a 

 magnetic needle to the right or left. In others, 

 again, they are given by mechanism connected 

 with an armature of an electro-magnet. It sways 

 to and fro under the action of the electro-magnet 

 and a counter-spring, or between two electro- 

 magnets. 



Indicators of all kinds are either such as give 

 merely a passing signal to the eye or ear, or such 

 as permanently record the signals. Cooke and 

 Wheatstontfs needle telegraph is the best known of 

 the former class, but it is now little used except 

 in this country. At present, the form most widely 

 used is of the second class, Morse's recording 

 telegraph. It leaves, in point of simplicity and 

 accuracy, little more to be desired. We shall, 

 therefore, give the general arrangement of a 

 telegraph chiefly on Morse's system. 



Morse's recording instrument, or, as it is shortly 

 termed, the ' Morse,' or Register, is shewn in fig. 

 34. L is the end of the line-wire which brings the 

 current ; E is the end of the wire which passes it 



Fig- 34- 



off to earth, after it has traversed the coils of the 

 electro-magnet, MM'. When the current passes, 

 and MM' is magnetic, the soft iron armature, A, is 

 drawn down, and thus the opposite end, /, of the 

 lever //', to which it is attached, is forced up. 

 When the current ceases, and MM' can no longer 

 attract A, the spring, s, brings back the end /; and 

 as the current continues alternately to flow and to 

 stop, the lever will keep oscillating between the 

 stops m and n. At the end /, the lever carries a 

 steel point or tracer, p, which, by the upward 

 motion of that end, is brought against a strip of 

 paper PP'. Now PP' is carried towards P' by the 

 rollers r, r 1 , which are kept in motion by clockwork, 

 quite independently of electricity. As long, then, 

 as the pointer is pressed up, the paper strip is 

 made to rub against it, and a line is thus embossed 

 on its upper surface. To make sure of its doing 

 this, there is a small groove in the upper roller, 

 opposite the pointer. It is easy to see, then, 

 that if the pointer press up for a moment only, a 



