226 BELL SYSTEM TECHNICAL JOURNAL 



In the study to determine the extent to which this theory would 

 check under the varying conditions of natural winds only the case 

 where 7 = 0° (both supports at the same level) was considered. 



The camera used in this study was equipped with a Tessar F-3.5 

 lens with a nominal focal length of 40 mm. The films were analyzed 

 with the aid of a motion picture projector. In this projector the film 

 passed over a glass plate on which was engraved a linear scale graduated 

 in hundredths of an inch. The pictures, together with the graduated 

 scale, were projected on to a screen. This method provided a ready 

 means of determining the horizontal displacement of the wire images 

 on the film. The actual wire displacement was then determined 

 through the use of the following relationship: 



where Li = Distance from wires to camera lens, 

 L2 = Distance from camera lens to film, 

 Di = Spacing between wires, and 

 D-i — Spacing between wire images on film. 



The two wires were maintained at equal sags throughout this study. 

 The equation for determining the stretched sag of a wire if the supports 

 are assumed to be rigid is as follows: 



, , 3L ,_ „^ 3wL* _. 



.. + -(L-i?)a = g^. (3) 



where a — Stretched sag, 



R — Unstressed length of wire, 



L — Span length, ^ 



.4 = Cross-sectional area of wire, 



£ = Modulus of elasticity, and 



w = Resultant of wind pressure and gravity components. 



As explained in the text, even though the poles were strongly guyed, 

 the supports moved when the tension in the wires varied and cor- 

 rections were applied to the stretched sag to take account of this 

 movement. 



After determining the horizontal displacement and the stretched 

 sag of the wires the experimental angle of deflection (angle of equi- 

 librium position) was calculated by means of the equation: 



sm «' = -7- , (4) 



a 



