CHAMBERS'S INFORMATION FOR THE PEOPLE. 



local battery passes to the Morse, entering where 

 the line-wire was supposed to enter in fig. 34, and a 

 wire corresponding to E (in that figure) passes to 

 the metal pillar, P, of the relay. Clearly, then, a 

 current from the local battery will pass through the 

 Morse, whenever a current from the distant station 

 passes through the relay, and the duration of the one 

 will exactly correspond to that of the other ; for the 

 local circuit is complete only so long as ef is drawn 

 down on S. Thus, a current far too feeble to affect 

 the Morse is sufficient to make the interruptions 

 of a local circuit ; and the result is precisely the 

 same as if the Morse printed directly under the 

 action of the line-current. 



How several Stations are connected in one 

 Circuit. There are three ways in which a series 

 of telegraph stations may be joined, so that each 

 may send a message to another, and so that any 

 one may send a message to all the rest at the 

 same time. 



The first method is called the open circuit, 

 because the circuit is not completed except when 

 the handle of any key is pressed down. Each 

 station has a battery, which is brought into circuit 

 by pressing down the key at that station. All the 

 other batteries are out of circuit, and the current 

 passes from the line-wire to the key, thence 

 through the relay, and thence to the line-wire 

 again, at each station without interruption. 



Thus each may, if necessary, print a copy of the 

 same message at one and the same moment. The 

 interruptions of the current at the same instant 

 in the different relays open and close the local 

 circuits which include the Morses. 



Second, there is the closed circuit method, by 

 which the current constantly flows through all the 

 keys and relays, while no one operates. A single 

 battery at one of the terminals, or, better, one at 

 each end, is quite sufficient for the whole series. 

 The circuit is broken by any one key being pressed 

 down. 



A third method is adopted for long lines. It is 

 called translation. The two methods of open and 

 closed circuits are only for short lines of not more 

 than 200 to 300 miles. When two stations, 500 

 miles or more apart, are communicating, the 

 transmission is generally broken at one or two 

 stages on the way, and retransmitted. This fresh 

 transmission is done mechanically, without requir- 

 ing any attention or hand-labour. The lever of 

 the Morse is made to act as a relay to open and 

 close the current transmitted from afresh battery 

 at its station. Thus, the Morse may register a 

 copy of the message itself, while it serves at the 

 same time to send it on to the next stage. All 

 the intermediate stations may, of course, print 

 copies of the message at the same time. It is in 

 this way that parliamentary news is transmitted 

 from London to all the important towns between 

 it and Edinburgh or Aberdeen. 



Cooke and Wheatstone's Needle Telegraph is 

 the most common of non-recording instruments. 

 It is nothing but an upright astatic galvan- 

 ometer, with the needles loaded, to keep them 

 vertical when at rest. The deflections of the 

 needle to right or left, by the passage of the 

 current through the coils in one direction, or the 

 reverse, constitute the signals. All that is seen 

 of the communicator outside the instrument is a 

 handle under the dial. This handle turns a 

 cylinder inside, which acts as a commutator or 



280 



current-changer. When the handle is upright, it 

 is in the receiving position, and allows the current 

 from the distant station to pass to the coil of the 

 galvanometer. But when the handle is turned to 

 one side or another, it shuts off communication 

 of the distant battery with its coil, and puts its 

 own local battery in connection with the line-wire, 

 and therefore with all distant stations whose com- 

 municators are in the receiving position. The 

 needles deflect to right or left accordingly, both 

 at the sending and at the receiving station. 

 A combination of these right and left deflections 

 is used as an alphabet. Thus, A is made by two 

 left deflections ; B, by three ; M, by one right ; 

 R, by one left, then one right ; and so on. This 

 is the old way. Now, however, it is usual to 

 adopt an alphabet corresponding to Morse's. A 

 turn of the handle to the right, or of the needle to 

 the left, corresponds to a dot ; and a turn to the 

 opposite hand, to a dash. (See fig. 45). 



No relay is needed in this case, as the galvan- 

 ometer needle is sensitive enough to be worked 

 directly by the line-current. Several stations are 

 connected on the open circuit arrangement. 



There are many other forms of indicator that we 

 have not space to describe. Perhaps the most 

 wonderful contrivances in this branch of tele- 

 graphy are the Type-printing Telegraphs. That 

 invented by Hughes of New York, in 1859, was 

 one of the marvels of the last Paris Exposition. 

 It actually prints the message in Roman char- 

 acters on a strip of paper. Its construction is rather 

 complicated, but others of simpler character have 

 lately been invented. 



Submarine Telegraphy. The subject of ocean 

 telegraphs has of late years assumed the greatest 

 importance. From the first unsuccessful attempt 

 at cable-laying, which was made in 1850 between 

 Dover and Calais, to the achievement of laying 

 the Atlantic cable in 1866, was a great stride in 

 so short a time. Many difficulties, seeming at 

 one time almost insuperable, have been overcome, 

 and now the manufacture and management of 

 cables are all but perfect. The points to be secured 

 in their construction are mainly three. First, the 

 insulation must be perfectly faultless, for the 

 smallest flaw is infinitely more serious than in the 

 case of land-lines. Second, the cable must be 

 strong enough to bear the weight of a consider- 

 able length of itself, and to resist chafing on rocks 

 and inequalities of the ocean-bed. Third, the 

 very best conducting-wire must be used for the 

 transmitting core, on account of the extremely 

 feeble current which reaches the end of a long 

 cable, even when the insulation is complete. 



The construction usually adopted to satisfy 

 these requirements will be understood from 

 fig. 38, which shews 

 (full size) the ' make ' 

 of the Malta and 

 Alexandria cable. 



The core, C, is a 

 strand of seven 



copper wires, em- 

 bedded in an in- fijjj 

 sulating substance, 

 called Chatterton's Fig. 38. 

 Compound (= i part 



Stockholm tar, I part resin, 3 parts gutta-percha). 

 A strand of wires, though not so highly conduct- 

 ing as a solid wire of the same diameter, is 



