Masterpieces of Science 



an instructive manner the phenomena of tele- 

 graphy over long aerial lines. The two ends of 

 a cable may be in regions of widely diverse 

 electrical potential, or pressure, just as the read- 

 ings of the barometer at these two places may 

 differ much. If a copper wire were allowed to 

 offer itself as a gateless conductor it would 

 equalize these variations of potential with serious 

 injury to- itself. Accordingly the rule is adopted 



Fig. 61. Reflecting galvanometer 

 L, lamp; N, moving spot of light reflected from mirror 



of working the cable not directly, as if it were a 

 land line, but indirectly through condensers. 

 As the throb sent through such apparatus is but 

 momentary, the cable is in no risk from the strong 

 currents which would course through it if it 

 were permitted to be an open channel. 



A serious error in working the first cables was 

 in supposing that they required strong currents 

 as in land lines of considerable length. The 

 very reverse is the fact. Mr. Charles Bright, 

 in Submarine Telegraphs, says: 

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