558 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



connections are properly made. Not all methods of wrapping or all 

 types of terminals are equally satisfactory and it is the purpose of this 

 paper to describe investigations that have been made to determine the 

 necessary conditions for the best wrapped terminal. These investiga- 

 tions include a photoelastic investigation of the stresses in the terminal 

 and a photoplastic investigation of the strains in the outside wrapping 

 wire. A new photoplastic material, polyethylene, has been used which 

 has a stress strain curve similar to a metal and a birefringence propor- 

 tional to the strain. The use of this material makes possible the evalua- 

 tion of strains in the plastic region and may find applications in other 

 plastic flow problems such as the extrusion of metals. 



Even after such terminals have been satisfactorily made, there re- 

 mains the question of whether they will have sufficient life to satisfy 

 the requirements of the telephone plant. A design objective for most 

 relays and other switching apparatus of the telephone plant is an unin- 

 terrupted trouble-free life of forty years. Hence, unless the connections 

 are to be the limiting factor in the maintenance of the equipment, they 

 also should have a minimum life of forty years under the conditions 

 for which the apparatus is designed. In order to investigate the probable 

 length of life of such connections, theoretical and experimental work on 

 stress relaxation in metals has served as the basis for calculations and 

 t€sts. These have been extended to the materials and conditions of the 

 wrapped solderless connection and the results indicate that the life 

 should be adequate even under very severe ambient conditions. 



PHOTOELASTIC ANALYSIS OF STRAINS IN TERMINALS OF THE SOLDERLESS 

 WRAPPED CONNECTION 



In studying the conditions necessary to insure a good solderless 

 wrapped connection, it is desirable to know what strains occur in the 

 terminals and in the wrapping wires and how these vary with the ter- 

 minal shape, the winding force and other variables entering into the con- 

 struction of the connection. While some of these strains can be surmised 

 from the winding conditions and the shape of the terminals, it is difficult 

 to obtain any quantitative results by calculations on account of the fact 

 that the desirable terminal shapes are rather complicated and because 

 a good many of the strains are in the plastic region. 



To remedy this difficulty, use has been made of a photoelastic and a 

 photoplastic technique. For the inside terminal, all the strains, except 

 at the corners where the wires make contact with the terminals, are 

 elastic and can be approximated with an ordinary photoelastic tech- 



