44 SUBMAKINE CABLE LAYING AND KEPAIEING. 



small area, assuming that a safe thickness for mechanical 

 strength and insulation of dielectric have been provided for. 



The following determining factors are therefore necessary to 

 be known in addition to the KR and the length. Any one of 

 these factors being specified, the others can be found and all 

 required dimensions and data of the proposed core got out. 



1. The minimum resistance of the dielectric. 



2. The maximum weight or minimum resistance of copper. 



3. The minimum thickness of dielectric. 



4. The weight ratio of copper and gutta-percha. 



When any one of the above factors are specified together 

 with the KR and length, the dimensions and all data pertaining 

 to the core can be got out. We shall here consider the deter- 

 mination of core dimensions to satisfy given speed conditions 

 having given any one of the above four series of data. 



When the first or second determining factor is specified, it 

 is necessary to know the mean temperature of the sea on the 

 proposed route as correctly as possible. Where there is no- 

 existing cable on this route, of course the thermometric 

 surveys available have to be depended upon, but the network 

 of cables is now so general that in almost all cases th& 

 temperature can be determined electrically. This is done 

 by measuring the conductor resistance of a cable laid over 

 or near the proposed route, and deriving the sea temperature 

 from this result, the known resistance at standard temperature 

 and the coefficient for rise in resistance of copper per degree 

 of temperature. The temperature coefficient for high-conduc- 

 tivity copper, as produced under present conditions and over 

 the range of temperature to which cables are usually exposed, 

 was re-determined with great care by the eminent firm of 

 civil and electrical engineers, Messrs. Clark, Forde and 

 Taylor, in the year 1899, and the true formula for degrees 

 Fahrenheit found to be 



^ = 14- 0-0023708(< - 32) + 0-00000034548(< - 32)^ 



•"32 



where R< is the resistance at temperature t Fahrenheit and Rs^ 

 is the resistance at temperature 32°F. 



In the pamphlet issued by this firm, entitled '* Temperature- 

 Coefficients of Conductivity Copper," dated February 20th, 

 1899, it is stated that, using the above formula for calculating 



