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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



resented by the wire or conductor ; and the re- 

 turn part, represented by the earth itself; and, 

 inasmuch as these two parts must be kept dis- 

 tinct and apart throughout their length, the wire 

 which is laid along the earth's surface must be 

 kept apart from the earth, to secure which the 

 conductor is entirely surrounded with an insu- 

 lator. In land-lines, erected on posts overhead, 

 the wire is separated or insulated from the earth 

 by the air, which is, when dry, the most perfect 

 insulator known ; and at the points of support 

 contact with the earth is prevented by the use of 

 porcelain, stoneware, or vulcanite insulators, to 

 which the conducting-wire is fastened. 



An insulator is a non-conducting substance, 

 impervious, so to speak, to electricity. It is the 

 theoretical antipodes of a conductor. While the 

 conductor is a substance through or over which 

 electricity flows freely, the insulator will neither 

 permit electricity to pass through its mass nor 

 over its surface. It can, therefore, be used as a 

 means of confining electricity to a conductor, 

 and preventing it from escaping to other con- 

 ductors in the neighborhood. In short, it can be 

 made to insulate or isolate the particular con- 

 ductor from all other conductors. Its use in a 

 submarine cable is to confine the electric current 

 to the conductor or wire, so that it travels along it 

 from one station to the other without escaping to 

 the water, and through that to the earth (which, 

 as we have already said, is the neighboring con- 

 ductor, and the return part of the circuit) on its 

 way. It is, therefore, of course, important that 

 there shall be no flaw in the insulator; and, in 

 order to protect it from strain and violence, it is 

 covered with' a strong guard or sheathing. This 

 outer sheathing or protector, which is composed 

 of twisted-metal strands, is purely mechanical. 

 Only the conductor and the insulator are con- 

 cerned in the electrical requirements of the cable. 



The conductor is invariably of copper wire, 

 that metal being chosen because, next to silver, 

 which is, of course, too expensive, it is the best 

 metallic conductor of electricity. The metals, 

 as distinguished from most other minerals, are 

 excellent conductors of electricity ; that is to 

 say, they oppose relatively less resistance to the 

 passage of the electric current through their 

 mass. There is an economy of power in using a 

 good conductor for the telegraphic line. The 

 current is less weakened when the resistance to 

 its passage along the line of wire is less, and 

 it is, therefore, capable of more powerful effects 

 throughout the route, and consequently at the 

 other end. The conductivity or conducting 



power of a wire increases with the thickness of 

 the wire ; and, therefore, by taking a thicker 

 wire of a more common metal than copper (such 

 as iron), the resistance to the passage of the cur- 

 rent may be made as small as when a thin cop- 

 per wire is employed. But it is important that 

 the conductor of a submarine cable, especially 

 of a long one, should be of as fine dimensions as 

 possible, in order to economize insulating mate- 

 rial and sheathing, and reduce the total weight 

 of the finished line. Therefore the advantages, 

 in point of price, of iron wire over copper, in 

 the first place, would be greatly overbalanced by 

 the increased cost of insulating and sheathing it. 

 It is of the greatest importance that the copper 

 wire of the cable should be as pure as possible, 

 for the slightest trace of arsenic or other foreign 

 element is sufficient to hamper, in some mys- 

 terious way, the swift course of the subtile cur- 

 rent, and very materially to weaken the conduct- 

 ing power of the wire. 



In a few cables the copper conductor has 

 been made in the form of a single thick wire; 

 but, for the sake of greater flexibility and less 

 risk of breakage, it is generally made in the form 

 of a strand of three or more, and frequently of 

 seven wires — six set round a central one. The 

 wires are wound together in a spiral strand, and 

 their interstices filled with an adhesive substance 

 called Chatterton's Compound, a mixture of resin, 

 gutta-percha, and Stockholm tar. This com- 

 pound not only renders the strand solid and im- 

 pervious to water, but also acts as an adhesive 

 connection between the copper conductor itself 

 and the insulator with which it is to be coated. 

 Bound together with this or similar pitchy com- 

 pounds, the conductor and the insulator form a 

 solid core expanding and contracting together. 



The insulator is always either of gutta-percha 

 or India-rubber, but most frequently the former; 

 and it is, of course, essential that there shall be 

 no flaw or defect, such as an air-bubble or steam- 

 vesicle, or hair or thread inclosed so as to de- 

 teriorate its insulating properties. To guard 

 against such accidents, it is usual to apply a se- 

 ries of coatings to make up the total thickness 

 of the insulator. Accordingly, when one coating 

 has cooled, a layer of Chatterton's Compound is 

 applied to it, and another coating overlaid, and 

 so on, until the required amount of insulating 

 material has been put on. 



Whether the insulating substance is gutta- 

 percha or India-rubber, there is generally wound 

 round it a serving of untarred hemp or jute-yarn, 

 which has either been tanned or soaked in brine 



