ELECTRIC DISTRIBUTION AND WIRING. 



283 



/, from each other, d being the difference in level of the tops of the poles, and //"the 

 sag of the wire below the top of pole, A, as shown. The given span, AB, may be con- 

 sidered as part of a longer span, AC, of which the length is L, as shown in the fig- 

 ure ; and the portion, BD, of the given span may be looked upon as a span also. 

 Let S be the length of wire in the long span, AC, and P the length of wire in the 

 short span, BD. Then the length of wire in the given span, AB, is : 



-f+i <'> 



Furthermore, applying equations (39) and (40) to the span, AC, we have 



and 



8/7 



Applying equation (40) to the span, BD, gives : 



-(=.' 



3 (2l-L) 

 The equation to the parabolic curve formed by the wire is 



(iv) 

 (v) 



(vi) 



and at the top of the pole, B, y = H d, and y / Z/2 , so that : 



Equations (41) and (42) are obtained by eliminating L, P and S from (iii) and 

 (iv) by means of equations (v), (vi) and (vii). 



TENSILE STRENGTHS, WEIGHTS AND COEFFICIENTS OF EXPAN- 



SION OF WIRES. 



Usual factor of safety from 2 or 3 in warm climates to 6 or 7 in cold climates. 



Factor of safety for aluminum must be larger than for other metals on account of 

 low elastic limit of aluminum. 



Breaking tension of wire Tj, = ad 2 in pounds. 



Weight of wire w bd 2 m pounds per foot, where d is the diameter of the wire 

 in mils. 



Length at / F. = length at /' X [i + P(t '')] 



For weight of galvanized iron or steel wire add about 6 per cent, to weight of plain 



