IV, > 



UNDDLATORY FORCES. ELECTRO-MAGNETISM. [TUB NBBDLB OOIL. 



needle* will instantly diverge in a direction contrary to 

 that which they had previously done. 



We may sum up, as follows, the fact* taught us by the 

 last experiment, and those to which we have also referred 

 in our previous pages : for we learn 



1st That an electric current diverges a magnetic 

 needle, under certain conditions. 



2nd. That the direction of the divergence depends on 

 the direction of the electric current. 



3rd. That these results can be obtained at any distance 

 in two instruments, providing a sufficient conductor 

 intervenes between them. 



4th. That the divergence occurs at the same moment 

 (in almost every instance) in each instrument. Prac- 

 tically, there is scarcely any exception to this rule in 

 land telegraphs; and its occurrence in submarine tele- 

 graphs will be duly noticed. 



Having thus established the principle on which a 

 message may be conveyed, we shall now explain how the 

 divergence of the needle is used as a means of indicatm.' 

 the message sent by the wires. For the sake of sim- 

 plicity, we shall omit all other considerations as to mode 

 of conducting, obstacles, earth currents, ic., fee., until 

 we have made tho second question clear to our readers. 



2. THE DECIPHERMENT OF THE MESSAGE. We stated 

 that there are only two divergences from which the 

 message can be read, if one needle alone bo used at 

 each station ; and, for the present, we shall confine our- 

 selves to the examination of tho single needle arrange- 

 ment. It is obvious, however, that even with the 

 deflections of one needle, numerous signals may be trans- 

 mitted between any two stations, if such deflections are 

 taken singly or together. 



The following diagram illustrates the code which lias 



long been established ; and which, although at first si^'ht 



it may present some difficulty to the uninitiated, is, after 



all, very simple in its construction. The lines indicate 



mi IIXOLI XDL* the direction in which each divergence 



ALI-UAKT. of the needle takes place ; and the 



\ i letters next them are those repre- 



r~ \ Ml sentcd by the deflection. The small 



,. .. lines, in some signals, indicate that 



A \\ N // a combination of deflections, in dif- 



... ... ferent directions, is used. Thus, two 



B \\\ O /// to the left represent the letter A ; 

 .... .... whilst one to the left and one to tho 



C\\\\ P /I/I right V represent the letter D; and 



. .so on. For the annexed, and some 



D v R J of our future illustrations, we are 



. . indebted to Mr. Shaffuer's work on 



E \ v S Jl the Electric Telegraph 



It is thus evident that this com- 

 F\\v T Jll bination of deflections may be em- 

 ployed to spell out any message 

 Q \, U J which may be required to be sent 

 from one station to the other. It is 

 II \V V J/ true that such a simple arrangement 

 as we have described in our previous 

 I \. ^ ^ P*!? 88 !* would be quite unfit for this 

 purpose; because, the needle, after 

 X IJ having been once deflected, would 

 require a length of time before 

 Y JJ K am i n 8 i* 8 primary polar position. 

 At the same time, our readers may 



L \A 7 "VV. cas ''- v * r y * e ^ ec ^ th deflections in 

 " a leisurely manner, and so gain a 



practical knowledge of the working of tho more perfect 

 kind of instruments, which wo shall presently describe. 



\V-: have gone so far as to show, that a message, or a 

 series of signals, may be transmitted, by means of a con- 

 ducting wire, from one place to another ; and that such 

 signs may, by a preconcerted arrangement of signals, 

 be subsequently deciphered, and rendered available for 

 telegraphic purposes ; the deflections of the needle, in any 

 specified direction, being employed for that purpose. \\ 

 shall now advance another step, and explain the instru- 

 ments which are actually used; commencing with the 

 single needle telegraph, and afterwards describing the 

 Be* anlt, p. 149. 



double needle, which is most generally adopted throughout 

 Great Britain. 



Instead of a needle floating on the surface of a liquid, 

 one is suspended on pivots in a suitable frame ; and, over 

 this, wire is coiled, so as that each convolution may be 

 parallel, and in as close proximity as possible, to the 

 magnetised needle, in the manner illustrated in the 

 following engraving. 



r\t- 21. 



In the above cut, the needle is enclosed between two 

 coils of fine covered wire, the terminals of which nro 

 connected with two binding-screws. The wire is coiled 

 on a frame; and the more numerous the convohr 

 the greater effect will the current produce on the n 

 itself. The reason of this we have already explained 

 when wo described the arrangement of the ordinary 

 electro-magnet, t 



The external needle is simply an index, which oscillates 

 to and fro before a card on which the symbols or deflec- 

 tions are represented ; and thus the person watching the 

 instrument can at once see the deflections of the noodle, 

 and the code of signals which he has to follow. 



As wo are anxious to make our readers practically ac- 

 quainted with the working of the telegraph, we shall 

 suggest the construction of a model, which, for a trilling 

 expense, will enable them to actually practise the tele 

 graphic code. The following engraving illustrates a 

 simple instrument, which we have often used for public 

 illustrations of tliis subject. It consists of an upright 

 board, on which is fixed a 

 small coil of covered copper 

 wire, to the ends of which are 

 attached two binding-screws, 

 at the foot of tho instru- 

 ment. Inside the coil a 

 magnetised steel needle is 

 suspended on pivots, so that 

 t may freely move right or 

 eft. It should be hung a 

 ittle out of its centre, so 

 :hat tho lower part may, by 

 its weight, restore the needle to a perpendicular position 

 after a signal has been sent, and the current withdrawn. 

 if these instruments are required ; and they can be 

 made at an expense of three or four shillings cm li. 



A wire must be attached to one of the binding-screws 

 of one instrument, and be extended to that of the other; 

 and a wire from each instrument is to be brought to 

 the battery, which may be a single cell of any kind. 



t See unit, p. MO. 



Fig. 25. 



