. The Bell System technical journal . ——-^ Z 1-OlU i:^ 3 / ^ / f PRESSURE 2000 LB PER SQUARE INCH TEMPERATURE 120° F UJ o 2 / / P HENOL . FIBRE 1 U ^ 0 f 24 48 72 TIME IN HOURS 96 120 Fig. 8—Flow-test results for hard rubber and phenol fiber. 244 BELL SYSTEM TECHNICAL JOURNAL shown in Fig. 7, so that the temperature throughout the 24 hour testmay be maintained at 120° F. corresponding to the maximum Hkely tobe experienced in service. The amount of shrinkage or flow is meas-ured on the dial previously shown. Fig. 8 shows the relative per-formance of hard rubber and phenol fiber under the condit
. The Bell System technical journal . ——-^ Z 1-OlU i:^ 3 / ^ / f PRESSURE 2000 LB PER SQUARE INCH TEMPERATURE 120° F UJ o 2 / / P HENOL . FIBRE 1 U ^ 0 f 24 48 72 TIME IN HOURS 96 120 Fig. 8—Flow-test results for hard rubber and phenol fiber. 244 BELL SYSTEM TECHNICAL JOURNAL shown in Fig. 7, so that the temperature throughout the 24 hour testmay be maintained at 120° F. corresponding to the maximum Hkely tobe experienced in service. The amount of shrinkage or flow is meas-ured on the dial previously shown. Fig. 8 shows the relative per-formance of hard rubber and phenol fiber under the conditions of thistest. Molded PlasticsIn recent years there has been great activity on the part of manu-facturers of molded plastics to develop improved molding compounds,and we have endeavored to keep informed of new developments byexamining new compounds as they became available. An interestingproblem presented itself in the application of suitable molding com-pounds to a device known as a test strip, shown in Fig. 9. It will be. Fig. 9—100 point test strip used in switchboards. seen that it consists of a number of metal terminals mounted flush onthe face of the strip and projecting at the back to provide soldering lugsfor the central office wiring. In operation it is necessary to touch ametal contact plug to the appropriate test strip contact which will pro-duce an audible signal in the operators receiver. In passing the plugover live terminals an arc is drawn out, which is accentuated by ahabit of some operators of running their pencils along the groovesleaving a conducting path. Such arcs caused permanent conductingpaths in the surface of the bakelite, despite the adoption of strenuouscleaning routines. The need for a better insulating material for thisuse became even more urgent with a demand for a test strip having 200terminals Instead of 100 in the same space. Studies of compounds having such base materials as cellulose-nitrate,shellac, hard rubber, casein, and cel
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