254 



NATURE 



\7uly 29, 1875 



THE FROGRESS OF THE TELEGRAPH * 

 IX. 



In all submarine cables the copper conductor is com- 

 posed of seven small wires stranded together, an arrange- 

 ment which gives much greater flexibility and strength 

 than if a solid wire were employed. The general arrange- 

 ment of the signal apparatus in connection with the cable is 

 shown at Fig. 38. A, the battery, consists of a series of 



cells of Daniell's arrangement; B, the contact keys for 

 passing the positive and negative currents into the 

 cable ; C, " switch," placing the cable in connection either 

 with the earth, instrument, or battery as required ; D, a 

 form of Sir William Thomson's reflecting galvanometer 

 placed in connection with the cable by switch c ; E, the 

 permanent magnet arrangement for steadying and 

 adjusting the coil-mirror (shown in section and detail, 

 Fig- 39) ; I, resistance coils interposed into the circuit 



i H ^ 



Fig. 38. — Genera arrangement of apparatus for working a submarine circuit. 



t 



between the instrument and the earth ; j, a switch 

 for connecting the line to earth ; F, a darkened re- 

 cess to receive the scale upon which the spot of light 

 reflected from the lamp situated behind the partition, the 

 ray from which, passing through a slit in the direction R 

 is reflected back from the galvanometer mirror in the 

 direction r' ; the spot of light moves to the right or left of 

 the zero on the scale, according 

 as a positive or negative cur- 

 rent is passed through the cir- 

 cuit ; the several signals being 

 indicated by the successive 

 oscillations of the luminous 

 image, signals which corre- 

 spond to the conventional iil- 

 phabet of the Morse system. 

 The Morse alphabet is given 

 at Fig. 40. 



A steam-engine without the 

 motive power, steam, is no- 

 thing but an arrangement of 

 iron levers, cranks, and 

 F18 39.— Section of coil of Thorn- throttle valves, useless so far 



son's mirror galvanometer, show- ^g actual WOrk is COnCemcd. 

 iMg the mirror and magnetic t ti i i -u 



netdle suspension. I" like manner a telegraph 



instrument without the electric 

 current to actuate its parts and give vitality to the circuit 

 is valueless — a piece of apparatus to be inspected on a 

 museum shelf. A few remarks upon "Batteries" are 

 therefore necessary before an examination is made into the 

 chief laws which regulate the passage of electric currents 

 through metallic conductors. 



* Gontitiuecl from p. 151. 



In the production of a galvanic, or voltaic current, two 

 conditions are essential, either the presence of two metals 

 and a liquid, or two liquids and a metal. This will be 

 explained by reference to everyday phenomena. 



a 



a - — 

 b —- 



n 



J -~ 

 k — - 



1 - — 



o 

 P 



q 



r 

 s 

 t 

 u 

 li 



V 



w 



X 



y 

 z 

 ch 



Fig. 40.— The Morse Alphabetical Code. 



A familiar example of the development of an electric 

 current by two metals and a liquid is continually pre- 

 sented to our notice in the wasting of the iron bars of 

 a railing close to their junction with the stone coping. 

 Here we have the two metals, the iron composing the 



