HI 



ULATORY FORCES.-ELECTRO-MAONETISM. [EARTH COXDI-CT:OX. 



that they may bo tested for any defect This precaution 

 U, of course, unnecessary when the wires are iu*l . 

 in the air, bocauM any fault can be at once both soon 

 and renu- . 



Wo must here remind our readers, that we have 

 hitherto supposed that the circuit, or course of the 

 electric current, U passed from one station to another, 

 and back again, by means of a continuous or double con- 

 ducting wire; but such U not done iu practice. The 

 earfJi is made to act as a conductor ; and thus the expense 

 of a second wire is saved. It is n moot singular fact, that 

 moist earth acts just a* well as a solid wire in this respect ; 

 and if any number of instruments bo working at the same 

 time, ana each using the earth for the purpose referred 

 to, (till each receives and indicates its message without 

 the least confusion, providing an independent wire also 

 joins the separated but corresponding instrument. 



The following engraving illustrates the mode in which 

 the earth is thus employed. 



Fig. 27. 



In the above engraving, a a a represent the conducting 

 wire suspended on the poles, bb; c and d are two tele- 

 graph instruments, each of which is connected with the 

 wirej a ; t, and / are wires which lead respectively from 

 the instruments e and d; and at the ends of each of 

 these wires, plates of metal, g and h, are affixed, which 

 are buried at some distance beneath the surface of the 

 ground, in moist earth. Now, supposing a current is 

 passed from the station c, to a distant one, d its 

 course will be by the wire a a a, suspended on the poles ; 

 proceeding from c until it reaches d ; and thence leaving 

 the instrument by the wire, /, it passes into the earth by 

 the plate, h : from this it then returns in the direction of 

 the arrows, until it arrives at the plate g ; by mcaus of 

 which it reaches the first station again by the wire e, 

 into the instrument, c. A continuous circuit is thus 

 established between the two, or, indeed, any number of 

 stations ; and, as we before remarked, it answers just as 

 well as if a second conducting wire were employed. 



Our readers will at once perceive the necessity which 

 exists for maintaining a complete insulation of the wire 

 from the earth ; and in this is found one of the difficulties 

 of telegraphing. It is almost impossible to keep the 

 wires and poles completely dry during wet weather, 

 although the glass or porcelain holders intervene. Now, 

 moisture is a conductor of electricity; and as such, a 

 portion of the current is often diverted from the wire 

 towards the earth, in wet weather, by means of the wet 

 polos. This circumstance is one, therefore, which in- 

 terferes botli with the speed of transmission and the 

 distinct indication of a message, when rain, fog, or dew 

 are prevalent. 



It has been already mentioned, that the electric currents 

 of the earth have a manifest influence on the magnetic 

 needle ; and in this is found another and serious source 

 of annoyance to telegraphers. We have frequently 

 noticed the needles of instruments, at stations far apart, 

 working with all the vigour which would bo seen it the 

 clerks were engaged at the moment in sending a message 

 by the wires. The needles oscillate in a most irregular 

 manner, and not with that decision of appearance which 

 is noticed when a message U actually being sent. The 

 currents of atmospheric and terrestrial electricity vary 

 constantly in their intensity ; and, in some cases, are of 

 extreme violence. Wo have noticed them as especially 



prevalent on the Edinburgh and Glasgow line, which 

 runs nearly duo east and west ; and have heard some 

 curious details of such occurrences, from the operators on 

 the instruments at the extreme stations. The same 

 effects, however, are experienced on all telegraphic lines, 

 in a greater or less degree. 



Atmospheric electricity, when present in excess as in 

 tlio case of a thunder-storm is another source of dif- 

 ficulty, and possibly of danger, to the operator; this, 

 however, is chiefly confined to lines in wluch the wires 

 are suspended on poles. 



In a great extent of country, of course, atmospheric 

 changes are constantly occurring; and, to some extent, 

 these changes may, by means of the conducting wires of 

 the telegraph, compensate with each other, so far as that 

 instrument is concerned. For instance we may readily 

 suppose, that in a distance of 40 or 50 miles, the 

 atmosphere may occasionally be charged, at different 

 places, with electricity of opposite characters ;* and if the 

 amount of these be equal, no effect would result on tliu 

 telegraphic wires extended between any two distant 

 stations. But it may so happen, that at or near one of 

 these, a local electrical disturbance may occur ; and such 

 is sure to be followed by derangement of the needles of a 

 telegraphic apparatus, if no provision be made to obviate 

 it. Sometimes the electricity, during a thunder-storm, 

 has passed by the conducting wires into a station, and 

 completely destroyed the whole apparatus; or, at all 

 events, so affected the needles as to render them useless. 

 At times their polarity is reversed ; and instances have 

 been met with in which the persons in attendance have 

 narrowly escaped with their lives. The remedy, however, 

 is comparatively easy of execution. It is unnecessary 

 for us to remark, that the intensity of the voltaic current 

 is infinitely less than that of the electricity set free 

 during a thunder-storm. For whilst the former requires 

 a continuous and good conductor, the hitter will readily 

 pass through a stratum of air, between two conductors, 

 and so reach the ground, if proper arrangements are 

 made for that purpose. This fact is taken advantage of 

 to prevent the accidents to which we have alluded. In 

 some instances the poles are provided with lightning 

 conductors ; a plan adopted by the London District 

 Telegraph Company. In others, temporary arrangements 

 are contrived ; so that, in the event of a thunder-storm 

 occurring near a station, the atmospheric electricity con 

 be readily directed to the earth, by means of a condm t< .r 

 of sufficient size; in which, however, a slight break is 

 mode enough to prevent the escape of a current pa> 

 from a voltaic battery. Some ingenious inventions li .\\<- 

 been applied for this purpose ; which, however, are of spe- 

 cial interest only to those engaged in telegraphic matters. 



It is a remarkable fact, that although the greater 

 portion of the telegraphic wires in this country are 

 suspended on poles, and entirely unprotected, it ran ly, 

 if ever, happens that they are subjected to intentional or 

 malicious injury. With all possible charity for the 

 qualities of human nature, wo can scarcely suppose that 

 the immunity of the wires is duo to the good feeling of 

 all classes, especially of the juvenile members. \\ e 

 believe, so far as our experience goes, that a general 

 impression exists, to the effect that a powerful shock 

 would be obtained on touching or meddling with the 

 wires; and most young people throughout the country, 

 have, at one time or another, made acquaintance with this 

 kind of electrical phenomena. There is, at all events, a 

 general opinion entertained, that birds flying .against the 

 wires are instantly "killed by the electric shock;" 

 although it has not the least foundation iu fact ; for it 

 would bo impossible for any creature to feel the slightest 

 electric;il inllucnce under such circumstances. This, 

 however, is an instance of many r.u-us in which ignorance 

 becomes a protection rather than a bane. Except in 

 extraordinary coses, storms of wind do comparatively 

 little damage to the wires; and as any accident can bo at 

 once reported by the guards and engine-drivers continually 

 passing, little or no hindrance ever arises from this ca 

 There are many other circumstances which tend to the 



8c ante, p. 18J. 



