223 APPLIED SCIENCE 



had been given to the Atlantic Company from the time of the 

 failure of the 1858 cable. 



The above description of Mr. Smith's invention is not strictly 

 accurate as applied to the arrangements used during the expedi- 

 tion, but the leading idea remained unaltered. Thus the Wheat- 

 stone balance was used to measure the insulation resistance in 

 definite units, instead of the simple deflection insulation test. 

 The bridge was arranged with what Sir William Thomson calls 

 a potential divider, a set of resistance coils giving 10,000 equal 

 subdivisions by the mere sliding of two contact pieces. Con- 

 tinuity is never lost, nor the resistance irregularly changed, in 

 these slides a considerable practical advantage. A special 

 galvanometer was introduced to test continually the constancy 

 of the ship's battery, without which constancy the potential 

 tests would have been much diminished in value. On shore 

 the potential produced by the ship's battery was measured 

 by two methods perhaps more accurate than the deflection 

 through Mr. Smith's galvanometer and large resistance. One 

 method, also suggested by Mr. Smith, was by discharges taken 

 from a condenser charged by the conductor of the cable ; the 

 second by an electrometer reading, which could compare the 

 potential of the cable with that of each of the 10,000 sub- 

 divisions of a slide similar to that used on shipboard. The 

 battery producing current through the coils of the slide was 

 on shore also maintained constant, or corrected by observations 

 on a special galvanometer. By these arrangements the ob- 

 server could obtain, in a simple form, the various elements 

 required for the immediate calculation of the distance of a 

 fault had one occurred. 



The speaking arrangements were also modified. Charges were 

 not actually withdrawn from the cable or put in at the shore. 

 The withdrawal of a succession of charges would have produced 

 an appearance alarmingly like a fault. Mr. Varley suggested 

 the use of a condenser attached to the cable on shore, by which 

 he induced slight positive or negative charges, which trans- 

 mitted the signals to the ' Great Eastern.' He, as it were, 

 instead of at each signal withdrawing a few drops of fluid from 

 our typical pipe, pushed the water a little way back in it, or 

 pulled it a little way on, and signalled by these impulses 



