413 



The equation (No. 2) R=r(l +0'00192) gives the value of the 

 resistance R of the copper wire at any temperature t+a in function 

 of the resistance r at any temperature a (Fahrenheit). The length 

 and temperature of any coil being known, the resistance of the 

 copper wire was thus at once obtained from the resistance of one 

 knot at 60, which was very carefully determined. 



Now let G=resistance of cylindrical coating. 



D= deflection called the continuity test. 

 d= deflection called the insulation test. 

 C= coefficient expressing the relative delicacy of the two 



galvanometers. 



BR= resistance of the battery. 

 Tj= resistance of the coil of first galvanometer. 

 T 2 = resistance of the coil of second galvanometer. 



Tfc r CtanPx(BR+T 1 + M) M 



ThenG=- tan j -BR + T 2 + $. . . . (3) 



G having been thus obtained in any desired units, S, the specific 

 resistance of the material, can be at once obtained by equation No. 1, 

 which appears from several experiments to give constant values for 

 S when calculated from cables of different dimensions. In extreme 

 cases, however, the influence of extra resistance would render the 

 formula defective, especially after continued application of the cur- 

 rent : thus the resistance of a foot-cube would be very different to 

 that of an inch-cube. 



The values of G for the covering of the Red Sea cable, after con- 

 tinued electrification for periods of one, two, three, four, and five 

 minutes, were calculated in Thomson's Absolute British Units, from 

 four sets of tests made specially for this purpose on four different 

 cables, each about 500 knots long. Tables containing the results of 

 these calculations accompany the paper. 



A Table is also given of the resistance of the Red Sea covering 

 after one minute's electrification, and after five minutes' electrification, 

 at each degree of temperature, from 50 to 75 Fahrenheit. This 

 Table was formed by means of the temperature curves described in 

 the first part of the paper : this Table is here annexed (No. 1). 



Similar Tables were given for the covering of the two experimental 

 coils mentioned in the first part of the paper. The coil composed 



