PRINCIPLES OP DESiaN. 45 



from the conductor resistance the mean temperature of a 

 laid cable, the result so obtained agreed to within xo°F. with 

 direct observations by verified thermometers over the same 

 route, the thermometric survey in this case having been 

 carried out with unusual completeness. The hitherto-accepted 

 Matthiessen's coefficient is shown to be considerably out, as 

 when the temperature was calculated from the observed re- 

 sistance by means of this coefficient the disagreement from the 

 actual thermometric observations amounted to as much as 

 3-3 deg. 



The Committee on Standards for Copper Specifications, 

 which was constituted in 1899 of representatives from the 

 [nstitution of Electrical Engineers, the General Post Office 

 and the principal manufacturers of rubber- insulated cables, 

 adopted (in section 5 of their report of December, ] 899) Messrs. 

 Clark, Forde and Taylor's temperature coefficient as above and 

 resolved that the average coefficient of 0-00238 per degree 

 Fahrenheit be the accepted standard for commercial pur- 

 poses. This average coefficient is now adopted as standard 

 for high-conductivity commercial copper, and will be> used 

 throughout this work. 



The accepted formula for the variation of the resistance of 

 copper wi6h temperature in degrees Fahrenheit is, therefore, 



Ei,=R,[l+O-OO238(T-0], 

 where Rr= resistance at the higher temperature T°, 

 and Ri= „ ,, lower „ t°. 



By this formula the sea temperature may be determined from 

 a measurement of the copper resistance of a laid cable. For 

 example, suppose the conductor resistance at 75°F. is known 

 from core teats at the factory to be 4,850 ohms. The observed 

 CR tested afcer laying is 4,470 ohms. Applying the above 

 formula we have 



4,850=4,470 [1 +0'00238(75 - i)] 



from which the sea temperature (t) is found to be 39-2°F. It 

 may be pointed out that as the coefficient 0-00238 is a multi. 

 plier, it remains the same whether the resistances are expressed 

 in B.A. units or legal ohms. 



