PBINCIPLES OF DESIGN, 63 



first, and then drawn down to the requisite size for the weight 

 required. This process rendered the metal hard and it was 

 afterwards annealed. In annealing, the surfaces between the 

 segments became oxydised and the metal brittle and stiff. 

 Consequently the conductor did npt possess sufficient flexibility, 

 although this construction had the advantage of increased 

 conductivity for given dimensions, and this form of conductor 

 did not displace or survive the twisted strand. 



The " solid strand " conductor first introduced by Messrs. 

 Siemens has been more successful and is largely in use. It 

 was first adopted in the Direct United States Cable of 1894. 

 In the solid strand there is a solid wire in the centre surrounded 

 by a number of small wires (generally 12) laid spirally as in a 

 strand (Fig. 34). It is really a development of the stranded 

 conductor as originally used. The seven-strand wire was six 

 round one and the solid strand is ten or twelve round one, the 

 centre wire being of larger size. This is practically as good elec- 

 trically as the segmental form and less trouble and expense to 

 make. It was adopted in the Anglo-American cable of 1894 and 

 in the Commercial Company's Atlantic cable, 

 and is now largely used on long cables of high 

 speed requiring a heavy conductor. 



In the earliest cables laid the conductivity 

 of the conductor was as low as 30 to 40 per 

 Fig. 34. ^q^^^ Qf that of pure copper. In the 1862 



, ^ , ^ Persian Gulf cable the conductor was 90 per 

 Strand Conductor. , , i 



cent., and copper used for electrical conductors 



has risen steadily in percentage, until at the present time it 



is possible to obtain it at 100 per cent. Matthiesen's standard. 



For cable conductors copper is now usually specified to be 98 



per cent, conductivity ; but a reasonable commercial margin is 



allowed — that is, if on test the copper resistance comes out a 



little higher than stipulated the core is not rejected, provided 



the capacity is sufficiently below the specified figure for the 



KR to be within the limit required. 



The wires to form the stranded conductor are delivered in 

 hanks, drawn to gauge, and samples are from time to time 

 tested to check the percentage conductivity of the metal. 



The percentage conductivity of a sample of wire is usually 

 measured by balancing against an equal length of wire of known 



