396 



SUBMARINE CABLE LAYING AND REPAIRING. 



only be strictly applicable whea the false zero is immediate — 

 namely, when it is the first throw observed on releasing the 

 battery key indicating less resistance. If there is no appreci- 

 able electrostatic or electromagnetic capacity in the cable or 

 bridge to disturb the galvanometer on opening or closing the 

 battery circuit, the rush of the spot in the first direction must 

 be due to the polarisation electromotive force existing in the 

 exposure circuit during the last moments of testing current 

 flow, and consequently the correct position of the spot to select 

 as zero is that position which in its first throw it endeavours to 



Time in Seconds. 

 Fig. 234 



reach. Accordingly, in the experiments which established the 

 law, the balancing resistance was reduced step by step until 

 this first throw was so far diminished as to be no longer 

 appreciable. 



" If there were no electrostatic or electromagnetic capacity 

 in the cable and bridge, the curve D S would represent the 

 discharge due to depolarisation. It would then be possible to 

 read to the first throw D of the needle, or to ' immediate 

 false zero ' ; but the electrostatic capacity discharge is super- 

 posed, and creates the discharge curve Y S. After a certain 

 interval the electros batic discharge is over, and the curves 

 coincide. It is of no use to take any position of the spot as 



