392 



NATURE 



{March i8, 1875 



found to multiply. The introduction of gutta-percha in 

 1850 and a more perfect knowledge in the preparation of 

 indiarubber as insulating mediums for cleclrical purposes 

 have been the means of establishing upon a commercial 

 basis electric communication between the chief empires of 

 the earth, have united the eastern and western hemi- 

 spheres by metallic highways of thought, threading the 

 trackless ocean with its mysterious depths. It is thus that 

 the primitive experiment of Wheatstonc in 1S44 has deve- 

 loped into the stupendous telegraphic undertakings that 

 encircle the globe, uniting with a common interest all 

 nations, creeds, and languages. 



The principal laws that regulate the transmission of 

 electric currents through metallic conductors are simple, 

 and may be briefly described with sufficient distinctness 

 to enable the general reader to grasp the intricacy and 

 magnitude of the science of electric transmissions. 



Leaving on one side the old accepted terms of con- 

 ductors and non-conductors, for the present purpose all 

 substances in nature must be regarded as able more or 

 less to conduct a current of electricity. 



As the various substances, gums, glass, wood, earths, 

 liquids, or metals are examined, it will be found that 

 some afford much greater facility for the transmission 

 of electricity than others ; consequently, if they are ar- 

 ranged according to the resistance offered to the current, 

 a hst somewhat similar to the annexed will be presented, 

 commencing with those of least resistance : copper, iron, 

 plumbago, sea-water, rain-water, snow, steam, moist earth, 

 oils, ice, phosphorus, porcelain, baked wood, dry paper, 

 hair, silk, mica, glass, v.'ax, sulphur, shellac, gutta-percha, 

 india-rubber. It therefore naturally follows that where 

 it is required to construct a system of submarine circuits 

 for the conveyance of electric currents from one place 

 to another, some metal, such as copper, is selected. Water 

 being also pretty high in the scale, it is essential that 

 to prevent leakage or loss of current in its passage 

 through the wire, a gum such as gutta-percha or india- 

 rubber, offering a high resistance, should be selected in 

 which to enclose the conducting wire and give proper 

 insulation to the circuit. It is therefore evident that the 

 perfection or freedom from loss or leakage in an electric 

 circuit is simply relative as regards the material em- 

 ployed, and insulation means the obstruction or resistance 

 placed in the way to prevent the escape of electricity from 

 the conducting wire. Various important phenomena 

 come into play in connection with the passage of an 

 electric current through an insulated circuit, which it is 

 necessary to explain in an elementary manner. Induc- 

 tion, or the production of an electric current moving in 

 an opposite direction to that of the current passing 

 through the insulated conductor, takes place in the adja- 

 cent medium to that of the insulated wire ; that is to say, 

 supposing an insulated metallic circuit — a submarine 

 cable — is fulfilling its duties in the tran mission of an 

 electric current throughout the metallic conductor, the 

 effect of that current will be to set up a second current in 

 the water moving in an opposite direction. This opposite 

 or induced current is well illustrated by the Leyden jar 

 familiar to everyone. The inside metallic foil represents 

 the copper wire, the glass separating the foils the insula- 

 tion of the cable, the external metallic coating the water 

 surrounding the cable. On electrifying the Leyden jar 

 the internal and external metallic coats become charged 

 with electricity in opposite states. The effect of this 

 induced current on submarine cables is to retard or pull 

 back the flow of the primary current, sensibly diminishing 

 the speed ol transmission as compared with that of a land 

 line of telegraph. On a land line with a single wire the 

 effect of induction does not take place, because the 

 metallic conductor, generally iron, recjuires no insulating 

 medium to enclose it, the air itself taking the place m 

 the insulator ; the wire requires to be insulated only 



at the points of support, the tendency of these being to 

 produce a leakage or weakening of the current from water 

 and other substances held in suspension in the air, im- 

 pairing the integrity of the contacts at the points of sus- 

 pension. Induction, however, takes place with land wires 

 under certain conditions, namely, when two or more are 

 suspended closely together ; a current through one wire 

 will then produce an induced 

 current in an opposite direc- 

 tion in an adjacent wire. 

 Induction increases with the 

 extent of surfaces of the cop- 

 per conductor and the insu- 

 lator with which it is covered 

 diminishing the speed of 

 transmission. 



Insulation may be ob- 

 tained by a very thin cover- 

 ing of the insulating medium. 

 Increase in the thickness of 

 the material only mechani- 

 cally renders the covering 

 more secure. The effects of 

 induction are decreased in 

 proportion as the insulating 

 substance is increased in 

 thickness, the conducting 

 wire remaining the same ; 

 with an infinite insulation 

 like the atmosphere, induc- 

 tion would cease. 



With insulated wires, ab- 

 sorption (inductive capacity) 

 takes place. No substance 

 in nature has yet been found 

 that will not absorb some 

 other element, force, or 

 matter in a greater or less 

 degree. Heat, light and 

 electricity, liquids, gases, and metals under varied condi" 

 tions are all alike susceptible, and will either be influenced 

 or retain in different proportions the various elements, 

 forces, or matters brought into juxtaposition with them. 

 At present it is only necessary to investigate the pheno- 

 menon of electrical absorption or inductive capacity. 

 Thus, when a current of electricity passes through an insu- 

 lated metallic circuit, certain known effects take place. Re- 

 sistance, which impedes the direct progress of the current ; 

 induction, or the setting up of a counter-current moving 

 in an opposite direction, and exerting, as it were, a pulling 

 back of the original current ; absorption, or the sucking 

 up into the substance of the insulating material of a 

 sensible integrant of the original current. Various insu- 

 lating gums, as gutta-percha and indiarubber, have dif- 

 ferent properties as regards the insulation or resistance to 

 the lateral escape of the electric current they enclose, 

 namely, the inductive effect in proportion to the insulation, 

 and the absorption of the current as it flows through the con- 

 ductingwireenclosed by them. Asasponge sucks up water, 

 so to a certain extent does the insulator of the submarine 

 wire absorb the electric current, the result being that, 

 instead of the current passed into the wire at one end 

 flowing through and emptying itself out at the other end 

 of the wire, the current will flow out and leave a residue 

 behind, an appreciable time being required for discharge 

 to clear the line. This absorption of the current leaves 

 the line clogged for the receipt of the next current, and 

 greatly interleres with the rapid transmission of currents 

 through insulated metallic circuits. It is therefore only 

 in short cables that the transmission of the current may 

 be considered instantaneous. In cables exceeding 150 

 miles in length, electric currents have a sensible duration. 

 {J'o be continued.) 



9 — Chaiging the Leyden jar 



