ELECTRIC DISTRIBUTION AND WIRING. 



28l 



span using equation (40). 

 ture, /, by the equation 



Then calculate the length of the wire at summer tempera- 

 , = ,' [i + /?(/-//)] 



in which /? is the coefficient of linear expansion of the wire as given in the following 

 tables. From the value of s, so calculated, the value of the sag, //, at temperature, /, 

 may be calculated from equation (40), and then finally the tension, T, at summer 

 temperature, /, may be calculated from equation (39). 



It is to be noted that as a line wire cools and shortens, its tension increases, so that 

 its thermal contraction is accompanied by an elastic elongation due to the increase of 

 tension ; but this effect is generally neglected in practical line calculations, inasmuch 

 as the error is always on the safe side, that is, the actual winter tension is less than 

 that anticipated in the calculations. 



Pole line on a grade. It is usual to make the horizontal component of the ten- 

 sion of the wire the same in value all along a pole line on a grade, so that the actual ten- 

 sion of the wire is slightly greater on the down-hill side than on the up-hill side of 

 each pole. The problem of determining the sag corresponding to a given horizontal 

 tension, and the problem of allowing for the effects of temperature are treated in the 

 same way as in case of a pole line on a level except that the following equations are 

 used instead of equations (39) and (40) : 



r= 



_ 



1 



V 1 





(42) 



in which T, /, w, and s represent the same quantities as in equations (39) and (40), 

 d is the difference in level between the ends of the span, and H is the sag of the wire 

 below the upper end of the span, as shown in Fig. 162. 



^///////////^///^^^ 



Fig. 161. 



Derivation of equations (^9) to (42). Consider a wire, Fig. 161, suspended 

 between two points, / and p> '. If the wire is nowhere greatly inclined the actual 



