ELECTRO-TELEGRAPHS 



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presenting its ends at the face turned toward us in the drawing. This bar becomes 

 magnetic, forms what is called an electro-magnet every time and as long as an 

 electrical current circulates in the wire ; and its ends become respectively north and 

 south poles. A narrow plate of iron, an armature, as it is termed, is mounted on 

 pivots in front of the ends or poles of the magnet ; it carries a vertical stem upon 

 which the hammer is fixed. Every time the iron bar is magnetic the armature is 

 attracted, and the hammer strikes the bell. The spring or contact-maker for intro- 

 ducing the current of electricity into the circuit, is shown in front on the right-hand 

 side. This is Mr. "Walker's bell for signalling railway trains from station to station. 

 The language consists of one or more blows. One, two, and three blows are the signals 

 for common purposes, half a dozen blows is the limit. The acknowledgment of a 

 signal is its repetition. By a simple arrangement of an index, that moves in fellow- 

 ship with the hammer, the eye, as well as the ear, may read the bell signals. 



Fig. 808 shows another application of the direct action of an electro-magnet in pro- 

 ducing telegraph signals. It is Morse's printing telegraph, very generally used in 



808 





America, and to no small extent in Europe. The coils of wire are shown at 

 M, the armature at H, fixed at one end of the lever p, which is itself carried on centres 

 at c. The range of motion here is small in order to produce rapid utterance ; it is 

 regulated by the screws d and?'. The reaction of the spiral spring /restores the lever 

 to its normal position each time the magnetism ceases. The signals consist of dots or 

 dashes, variously combined, made by the pointed screw t upon the slip of paper P, 

 running from the drum at the right in the direction of the arrows ; a few such 

 signals are shown upon the end of the paper slip. "We have described the telegraph 

 proper, which is seen to be extremely simple. The only parts at all complex are, as 

 with the needle instruments already described, the mechanical parts, namely, the train 

 of wheels for carrying on the paper band, and the key or contact-maker, not shown in 

 the figure. The amount of pressure required from the point t in order to produce a 

 mark, is such that it cannot conveniently be produced by the magnetic attraction, 

 derived from a current of electricity that has come from a far distant station in order 

 to circulate in the coils of wire M. This difficulty does not prevail in the signal-bells, 

 fig. 807, which are, at most, not required to be more than eight or ten miles apart, and 

 in which also momentum can be and is accumulated so as to conspire in producing 

 the final result. Morse has, therefore, had recourse to a relay, as he calls it. This, 

 in principle, is pretty much the same thing as the instrument itself; but it has no 

 heavy work to do, no marks to make ; it has merely to act the part of a contact- 

 maker or key ; it can hence be made very delicate, so as to act well by such currents 

 as would not produce any motion in the instrument itself. The batteries which 

 furnish the electricity for doing the actual printing work in Morse's telegraph are in 

 the same station with the instrument itself. The office of the relay is to receive the 

 signals from afar, and to make the necessary connections with the local battery and 

 instrument so as to print off the signals on the paper in the usual way. It is obvious 

 that the motions of the instrument and the relay are sympathetic, and that what a 

 trained eye can read off from the one a trained ear can read off from the other. The 

 relays are constructed with much finer wire than is required for the instrument 

 itself, so that the current circulating in them, although very low in force, is multiplied 

 by a very high number, and becomes equal to the delicate duty required of it. 



It is, we hope, now understood, that, whatever form the needle telegraph may take 



