PRINCIPLES OF DESIGN. 



8^ 



senting the appearance in Fig. 41. The actual length of wire 

 which it takes to make one complete turn in a lay of I inches is 



sjc'^ + 1^ inches, 



where c is the pitch circumference in inches. The length of 

 wire in the spiral per unit length of cable is therefore 



x/^y 



+1. 



It is clear that this holds good whether the unit length is in 

 inches, feet or nauts, as the ratio of the length of a complete 

 spiral to the corresponding length of cable is the same whatever 

 length of cable is considered. For instance, if there were l^L 

 fathoms of each wire laid on per fathom of cable, there would 

 also be l^V nauts per naut of cable. Also the ratio is the same 

 whatever the number of wires in the sheath, and as weight is 

 proportional to length for a given area of wire or wires, the 

 above expression represents also the ratio of the weight of sheath 

 in one naut of cable to the weight per naut of the same number 

 of similar size wires considered as laid straight and not spirally. 

 Hence we can obtain from this the percentage increase in weight 

 of sheath due to lay. An example or two may make this clear. 



Light Intermediate Type. — Sheathing of 12 No. 9 wires : pitch 

 circumference I'Sia. ; lay 11 in. 



same 



Weight= V (i^y+ 1 = 1-02 times the weight of the 



wires laid straight. The increase of weight due to lay is there- 

 fore 2 per cent. 



The weight per naut of 12 No. 9 wires = 37 •44c wt. 

 Add 2 per cent. = 'TScwt. 



Actual weight of sheath = 38-2cwt. 



Heavy Sliore-End Type. — Outer sheathing of 36 No. 6 wires r 

 mean pitch circumference 6'15in. ; lay lOin. 



Weight =Vr^^^y + 1 = 1-13 times the weight of the 



same wires laid straight, that is 13 per cent, additional weight 

 for lay. 



