SOLDERLESS WRAPPED CONNECTIONS — PART II 571 



or if some regular shape such as a square, rectangle or rhombus is used. 

 Sharp corners are helpful since a sharp bend in the wire occurs around 

 them. 



2. The material of the terminal must be strong enough so that the 

 wire will not deform or cut through the terminal but must be plastic 

 enough so that an appreciable groove can be cut in it by the hoop stress 

 of the wire, in order that an air tight connection shall be made. The 

 most satisfactory metals are brass, copper, soft iron, nickel silver and 

 aluminum. 



3. In the most satisfactory solderless wrapped connection, the wrap- 

 ping wire unwinds to the extent of half a turn on each end and about 

 six turns or more are required to make a good connection. 



4. To maintain sufficient hoop stress, the constant wrapping stress 

 should be from 0.2 to 0.7 of the breaking stress of the wire. 



5. Shearing strains in planes parallel to the axis of the terminal 

 should not exceed 1 per cent in order to eliminate terminal set. 



PROTOPLASTIC ANALYSIS OF STRAINS IN WIRES OF A WRAPPED SOLDER- 

 LESS CONNECTION 



All of the strains in the inner block or terminal are elastic except at 

 the corners. Hence, for the interior terminal, photoelastic bakelite is a 

 satisfactory material for strain investigations. However, the outer wire 

 is necessarily stressed beyond its elastic limit and ordinary photoelastic 

 techniques cannot be applied. In order to see if the wire strains could 

 be studied with photoelastic bakelite, some time was spent in heating 

 rods to a temperature for which they become elastic, winding them 

 under a stress and cooling under the applied stress. Although the wind- 

 ing process was carried out successfully several times, the bakelite rod 

 always broke on cooling. This appeared to be due to the fact that bake- 

 hte is nearly linear up to the breaking point, i.e., it suffers from brittle 

 fracture and does not simulate a metal in this respect. 



Some measurements had previously been made at the Bell Labora- 

 tories and in England^ on polyethylene which indicated that it had 

 properties similar to a metal in the plastic range. Stress-strain curves 

 up to 15 per cent strain are shown by Fig. 14, and it is evident that on 

 the ascending part, the curve is very similar to that for copper or soft 

 iron. On the relief from stress, however, a considerably larger recovery 

 is obtained than for a metal. This material is fairly transparent and 

 the lower curve marked R/t shows the relative retardation for a 5461 A° 



2 Miss S. M. Crawford and Dr. H. Kolsky, Stress Birefringence in Polyethylene. 

 Proc. Phys. Soc, Section B, London, 6, Part 2, pp. 119-125, Feb. 1, 1951. 



