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BELL SYSTEM TECHNICAL JOURNAL 



If E is the modulus of elasticity in pounds per square inch cross-section, 

 D the diameter in inches, a the sag in feet and m the weight of wire per 

 linear foot, the approximate relationship^ is: 



a' + — (c - r)a = 



ttD^E 



As only horizontal winds normal to the line of supports are being 

 considered, the wind pressure when the loop is in equilibrium is 

 horizontal. The weight of the wire being vertical the two forces add 

 at right angles, their resultant being the square root of the sum of their 

 squares. This resultant lies of course in the plane of equilibrium of 



Fig. 6 — Test House and Line. 



the loop. The wind pressure component is about equal to the gravity 

 component for a velocity of 38 m.p.h. in the case of .104" wire and about 

 47 m.p.h. in the case of .165" wire. The effective weight of the wire 

 under these conditions would be greater by a factor of V2 than the true 

 weight. In general, ni in the above formula is the effective weight of 

 the wire per unit length. 



A wire having a sag of 5" in a 130' span with a temperature of — 10° 

 F. would have a sag of about 9" at 50° F. and about 16" at 100° F. due 

 to thermal expansion. The sag of such a wire would be increased by 

 wind pressure as shown in Fig. 5-A , the wind being given in true normal 

 velocity. The figure shows the increase to be most marked for low 

 temperatures and small diameters as would be expected. Similar 



^ Due to Mr. J. A. Carr of Bell Telephone Laboratories. 



