CABLE 



1036 



CABLE 



that are fast disappearing. His other writings 

 include The Negro Question; John March, 

 Southerner; Bylow Hill; and Kincaid's Bat- 

 tery. 



CABLE, SUBMARINE. When you read in 

 your morning newspaper about something that 

 happened across the ocean or on the other 

 side of the world only a few hours earlier, do 

 you ever realize that the telegraph under the 

 sea, which brought the news, is one of the 

 greatest wonders of this wonderful age? Your 

 grandfather probably does, for he can remem- 

 ber when weeks or months were necessary for 

 messages to come from beyond the seas. Of 

 course wireless telegraphy and wireless teleph- 

 ony are even more remarkable, but it is not 

 probable that they will ever make such huge 

 changes in the world as those the cable brought 

 about. Nor is it likely that they will entirely 

 supplant the cable, for wireless systems are 

 helpless during electric storms, while the tele- 

 graph beneath the ocean never fails. 



In 1864 a New York merchant wishing to 

 buy or sell in London had to wait twenty days 

 or more for an answer to each of his letters. A 

 year later only a few hours were required to 

 exchange cable messages. Thus the cable has 

 made possible an enormous trade between na- 

 tions, and has brought far-away lands like 

 Australia into close touch with Europe and 

 America. 



Cable Problems. To lay a continuous wire 

 along the bottom of the ocean from continent 

 to continent was by no means the only task 



HOW A CABLE IS MADE 

 (o) Outer wrapping of jute, etc. 



(b) Galvanized wire for protection against 



anchors, sharp rocks and flshes. 



(c) Jute wrapping. 



(d) Gutta percha or rubber. 



(e) Seven or more copper wires twisted to- 



gether. This is the part of the cable 

 which carries the message. 



The illustration at left shows a cross section 

 of a cable. 



The size of an ocean cable is not the same 

 throughout. In the deepest sea a cross-section 

 of the cable is slightly smaller than the one 

 shown above, but in the shallow water its diam- 

 eter is sometimes three times as great, for there 

 it is subjected to greater dangers. 



of the men who gave the world the first cable. 

 The electrical resistance in a wire 2,500 miles 

 long is enormous. (If the reader is not familiar 

 with the meaning of such words as resistance, 



he should refer to the article ELECTRICITY.) 

 But a strong current in a cable is apt to burn 

 the insulation and render the whole length 

 useless, so the current actually employed is 

 so weak that ordinary telegraph instruments 

 cannot be operated by it. A still greater dif- 

 ficulty was found in the fact that the outer 

 coat of protecting wires shown in the illustra- 

 tion makes the cable an electrical condenser 

 (an explanation of which will be found in the 

 article LEYDEN JAR), and stores electricity 

 which resists the passage of the message- 

 carrying current. Thus the first Atlantic cable, 

 completed in 1858, required over an hour to 

 transmit a message of ninety words from 

 Queen Victoria to President Buchanan, and 

 after a few weeks was permanently disabled, 

 presumably by too great strength of current. 

 The illustration explains in detail the con- 

 struction of the cable now in use. 



Instruments. The first man successful in 

 devising a receiver sensitive enough to be oper- 

 ated by the feeble current of the cable was 

 Lord Kelvin. This instrument was the reflect- 

 ing galvanometer. A concave mirror about 

 half the size of a ten-cent piece was hung by 

 a silk thread fastened to a brass ring above 

 and below. A magnetic needle attached to 

 the back of the mirror turned it back and 

 forth, according to the direction of the cur- 

 rent in two coils of insulated copper wire, and 

 the direction of this current was controlled 

 by the telegraph key of the operator on the 

 other side of the ocean. The mirror reflected 

 a spot of light on a scale three feet away; if 

 the spot was moved to one side a dot was 

 indicated, if to the other, a dash (see the arti- 

 cle TELEGRAPH for an explanation of the Morse 

 alphabet). 



Lord Kelvin later invented the siphon 

 recorder, which since 1867 has displaced the 

 mirror instrument. The siphon is a small 

 glass tube with its upper end in ink and its 

 lower end upon a strip of paper which passes 

 slowly along while the message is being re- 

 ceived. Like the mirror of the older appara- 

 tus, the siphon is moved to the right or left 

 by the magnetism of the current, but with a 

 greater degree of accuracy. But because the 

 magnetic action would not be strong enough 

 actually to drag the inked point back and 

 forth on the paper, the siphon is rapidly 

 vibrated up and down by an independent mag- 

 net, exactly in the manner of the hammer of 

 an electric bell. 



So many improvements have been made in 



