TELEGRAPHY. 



721 



TELEGRAPHY. Among the more recent 

 inventions and improvements in this branch 

 of applied science, the duplex and quadruplex 

 .systems of telegraphy, and the American auto- 

 matic telegraph, are the most important, hav- 

 ing already wrought a partial revolution in 

 this mode of sending intelligence, with prom- 

 ise of total and speedy displacement of the 

 comparatively slow and costly system now in 

 common use. The duplex telegraph, brought 

 for the first time into practical working shape 

 by Mr. Joseph B. Stearns, of Boston, makes it 

 possible to send two dispatches over a single 

 wire in opposite directions at the same time. 

 Attempts to do this have from time to time 

 been made ever since 1853, but without any 

 practical success until Mr. Stearns took up the 

 problem in 1868. It has now been brought 

 into use on several lines ; and it has also been 

 found, as will be seen when we come to speak 

 of the quadruplex telegraph, that the principle 

 which underlies the invention is capable of 

 very great extension. 



To understand the duplex telegraph it must 

 be borne in mind that the galvanic battery 

 gives birth to a force which returns in a circuit 

 to where it was generated, and accelerates the 

 liberation of more force, being, like a steam- 

 engine, employed partly in fanning its own fire. 

 This circuit can be performed much more ea- 

 sily through great lengths of some substances, 

 such as the earth and metals, than through 

 very small spaces of others, as the air and the 

 dilute acid of the battery. Galvanic electricity 

 is, therefore, strictly confined in a sort of mill- 

 round; or, for our present purposes, it may 

 best be represented by water flowing through 

 such a system of water-courses as is shown in 

 the annexed cut. We will suppose them to 



FIG. 1. 



include a reservoir and a secondary circuit at 

 each end. Let the reservoirs A and B have 

 water pumped into them by force-pumps, and 

 distributed by them to both the main and sec- 

 ondary circuits, in equal quantities and in the 

 direction of the arrows, so as to maintain the 

 water-wheels X and W in the same positions. 

 The highest points in the system must be sup- 

 posed to be at the front of the reservoirs, and 

 the lowest at the back of them. 



If an additional volume of water corne from 

 A, being equally divided on each side of TF, it 

 will not move that wheel, but it will move 

 VOL. xv. 46 A 



the wheel Xby destroying the balance which 

 previously existed there. But, if a similar ex- 

 tra volume be at the same time sent from B, 

 the pressure in that part of the circuit between 

 JFand JTwill overcome the opposing forces 

 at each of the points, and both wheels will be 

 worked, each virtually by the distant reservoir 

 and not by its own. 



If we substitute galvanic batteries for the 

 reservoirs, wires for the water- courses, and 

 electricity for the water, this gives us the prin- 

 ciple of the duplex telegraph. 



In the quadruplex telegraph, the invention 

 of Mr. Thomas A. Edison, of Newark, N. J., 

 the aim, as in the duplex, is to allow of several 

 persons using the same wire at one time. In 

 fact, the arrangement may be used as a duplex 

 telegraph if required, so that the wire is by it 

 made susceptible .of either double or quad- 

 ruple employ. 



The instruments used are modifications of 

 those of the Morse system. The "key" is 

 shown in Fig. 3, and the changes made to adapt 

 it to the uses of the quadruplex telegraph may 

 be understood from Fig. 2. The essential part 

 of the receiving instrument is an electro-mag- 

 net, which consists of a bent bar of soft iron, 

 surrounded at each end by a coil of wire con- 

 nected with the wire of the line. The current, 

 passing through these coils, communicates to 

 the iron core magnetic properties, and enables 

 it to attract another piece of iron or steel 

 called its armature ; but, when the current 

 ceases, the magnetism ceases also, and a spring 

 too weak to neutralize it draws back the 

 armature. It is shown in section at Jf, in 

 Fig. 2. When the armature and the lever 

 carrying it are discarded, and instead of them 

 a jointed tongue of steel, as at P M, is inserted 

 between the poles of the magnet, it will be 

 unaffected by the current except when a change 

 occurs in its direction. It is then called a po- 

 larized magnet. Its ' use will be explained a 

 little further on. One of the keys, JT, in the 

 diagram, is provided with a spring, which is 

 in contact with the metal of the key when this 

 latter is in its normal position, and maintains 

 across the key a circuit including a portion of 

 the battery V. But when the key is depressed 

 the spring comes in contact with a screw, to 

 which another circuit is connected, applying 

 the full strength of the battery to the line. 

 The circuit across the key is never broken, be- 

 cause the spring remains in contact with the 

 arm of the key until it begins to press against 

 the screw. This key works the magnet Jf, 

 which has its retractile spring so adjusted as 

 to be overcome only by the full intensity of 

 the current when the key is down. The other 

 key, J? 7 , is for changing the direction of the 

 current, and working the polarized magnet, 



