UNDULATORY FORCES. ELECTRO-MAGNETISM. 



259 



CHAPTER IV. 

 THE ELECTRIC TELEGRAPH. 



o explained the chief laws and facts in connection 

 with magnetism and electro-magnetism, together with 

 the lesser branch of magneto-electricity, we shall now 

 lay before our readers an exposition of the principles of 

 Electro-Telegraphy, and include a description of some of 

 the instruments which have been most generally adopted, 

 in different countries, for telegraphic purposes. 



It will be only necessary for us hero to call the atten- 

 tion of our readers to the remarks, experiments, and 

 illustrations which wo have already made in connection 

 with this interesting subject. We before stated, that wo 

 prr-iiime the student to have mastered the various facts 

 on which we have treated. This will prevent unneces- 

 sary repetition on our part, and will save both tiiao 

 and trouble to our readers. 



The earlier modes of telegraphy were of the simplest 

 description. Fires lit on the tops of high hills, nt dif- 

 ferent stations, served, by means of preconcerted signals, 

 to convey intelligence of any event to a great distance. 

 In more modern times, the semaphore answered a similar 

 purpose ; and even birds as the carrier-pigeon were 

 i-d to carry messages between correspondents. 

 is scarcely a doubt that our railway system has 

 been the chief means of perfecting the present mode of 

 telegraphy. Without it, it would be impossible that we 

 could attain that regularity and high speed, combined 

 with safety, which are the characteristics of railway tra- 

 velling and conveyance at the present time. In fact, 

 the discovery of Oersted laid dormant for nearly twenty 

 yean before it was used in its present application ; and 

 we are indebted to Messrs. Cooke and Wheatstone for 

 the perfection to which our telegraphic system has 

 arrivi-d. 



We shall not attempt to give any historical details of 

 the subject, as our pages are intended chiefly to convey 

 and explain facts and laws, rather than to uphold the 

 views, opinions, and interests of those persons to whose 

 ingenuity we owe so great a debt. Hence, we shall 

 only introduce their names and inventions as may best 

 suit our plan, leaving our readers to consult other works 

 which are specially devoted to the subject, and its nume- 

 rous and minute details. Wo shall commence with the 

 needle telegraph that being the most common form of 

 the instrument employed in this country; and, after- 

 wards, shall proceed to describe other arrangements 

 which are in use on the continent and in America. 



Wo have shown,* that when a current of voltaic elec- 

 tricity is passed, by means of a conducting wire, over 

 and parallel to a magnetic needle, the latter is diverged 

 ri^ht or left of its meridional plane, according to the 

 diivrtiuii of the current; and we also have seen, that 

 that direction may readily bo reversed, orj regulated, by 

 the most simple means. But whilst our readers will 

 easily understand the experiment and engraving which 

 illustrated our remarks, they may not be able to under- 

 Htand how the divergence of the needle, and the consequent 

 transmission of a message, is effected, when the station 

 to which it is sent is at a distance from the source of 

 power ; and, further, they will also have a difficulty in 

 finding out, how these divergences can be so managed as 

 to indicate the wishes or instructions which they are 

 intended to convey. It will be our business, therefore, 

 to explain both these points; and we shall thus, we 

 hope, in a clear and simple manner, remove all mystery 

 in reference to this most interesting subject. 



1. THK CONVEYANCE OF A MESSAGE. The first thing 

 required for conveying a message between any two sta- 

 tions, is a sufficient conducting medium, by means of 

 which the current of electricity can pass, so as to effect 

 a divergence of a needle, at a distant station which ftlui.ll 



Sco ante, p. 247. 



t Ante, p. 211. 



* A*te, p. 247 ; Hig. 3. 



agree with that produced at the place from which the 

 signal was sent. We cannot adopt a better plan to 

 explain the principle than that of an experiment ; and 

 shall therefore introduce a plan of illustration which we 

 have generally used, and which may be easily arranged 

 by the student. 



Experiment 21. Place, at the ends of a table, a basin 

 of water, and on the surface of the liquid of each, float 

 a magnetised so wing-needle, t Over, and parallel to the 

 needles, place a copper wire ; and, at either end of the 

 table, have ready a small voltaic battery of any sort in 

 action. Bring the ends of the wire close to the battery. 

 Now, so long as no current of electricity passes, the 

 needles will maintain thoir polarity, and sutler no 

 divergence. But if the ends of the wire are connected 

 with the zinc and copper of the battery, then a current 

 of electricity will pass from the copper plate, by the 

 wire, over the needles, and back to the zinc ; and, dur- 

 ing its passage, the needles will timttltaneously be 

 diverged from their previous direction, according to 

 that of the current, in a manner wo have alroady ex- 

 plained. X 



Now, it matters not how long the conducting wire may 

 be. For instance, the experiment may bo tried with one 

 basin and noodle, at one end of a garden, with the other 

 at any other place ; or from room to room in the house. 

 In its passage, the wire may be twisted, or fastened, in 

 any part, so that it be continuous throughout its length ; 

 because no mechanical action, like drawing or pulling 

 the wire, is required. All that is wanted is, that the 

 wire shall be of sufficient size to conduct the current 

 freely, and that the current itself shall bo sufficiently 

 powerful to traverse the entire length of the conductor. 

 The mode of carrying out this experiment is illustrated 

 in the following engraving. 



Fig. 23. 



I 



In Fig. 23, we observe each part of the arrangement. 

 The vessels a and b have, floating on the water, each a 

 magnetised sewing-needle. Over these, and parallel, 

 are portions of the wire, w w w, the ends of which are 

 I'onnrrtod with the zinc z, and the copper c, of a single 

 cell of a Banioll's battery. Any other form of battery, 

 or even a slip of zinc and one of coppor,|| will answer 

 equally ax well. 



After having thus noticed the simultaneous divergence 

 of the two needles in one direction, they .should then bo 

 allowed to come to rest first removing tho wi res from the 

 battery, so as to stop the passage of the electric current, or 

 the divergence of the needles would bo maintained. Tho 

 wires should then be changed as to their connoction with 

 the plates of tho battery. That which had !><< 'ii pre- 

 viously in contact with the copper Khould l>u applied to 

 the zinc, and vice vend. On this being completed, the 



1 Ant,, p. 188 ; Fig. 40. || An't, p. 18C ; Fig. 37. 



