560 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



specimen 0.4 inches square wound with nineteen turns of 0.050-inch 

 copper wire under a constant weight of twenty-eight pounds, which is 

 a stress of 14,300 pounds per square inch. It is evident from the sharp- 

 ness and number of the Hues that can be seen that we are deaUng with 

 a case of plane stress that can be analyzed by the method discussed 

 in the Appendix. Since the stress strain curve of copper wire has the 

 form shown by Fig. 4 with a yield stress* of about 26,000 pounds per 

 square inch and a breaking stress of about 34,000 pounds per square 

 inch, the appHed winding stress is about 42 per cent of the breaking 

 stress. Fig. 4 shows also the recovery measured for copper wire. The 

 recovery curves are quite accurately parallel to each other, but the larger 

 the strain the smaller the percentage recovery. Using the isochnic lines 

 shown by Fig. 24 of the Appendix and the method of analysis discussed 

 there, the stresses across and perpendicular to the line of ''eyes" of Fig. 3 

 are shown by Fig. 5. The stress perpendicular to the line of eyes meas- 

 ures the total compressive stress put on the terminal by the hoop stress 

 in the wire and from this measurement the average hoop stress remain- 

 ing in the wire can be calculated as follows. The cross section of which 



QUARTER-WAVE 

 ^'- PLATES --^ 



POLARIZER SAMPLE 



Fig. 2 — Elements of polariscope 



ANALYZER PHOTOGRAPHIC 



PLATE 



this force is applied is the width 0.4 inches by the length of the specimen 

 0.95 inches and hence the force applied by all the turns is 



F = 0.95 X 0.40 X 2000 = 760 pounds. 



(1) 



Since there were nineteen turns of wire wound around the specimen 

 and each turn has two sides exerting a tension on the bakelite, the aver- 

 age tension remaining in the wire, required to balance the compressive 

 stress, is 



T ^ 



760 



2 X 19 



= 20 pounds. 



(2) 



* In this paper tho. yield st ress is taken jis the point of greatest curvature of 

 the stress-strain curve. 



