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BELL SYSTEM TECHNICAL JOURNAL 



shown in Fig. 7, so that the temperature throughout the 24 hour test 

 may be maintained at 120° F. corresponding to the maximum Hkely to 

 be 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 this 

 test. 



Molded Plastics 

 In 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 by 

 examining new compounds as they became available. An interesting 

 problem 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 on 

 the face of the strip and projecting at the back to provide soldering lugs 

 for the central office wiring. In operation it is necessary to touch a 

 metal contact plug to the appropriate test strip contact which will pro- 

 duce an audible signal in the operator's receiver. In passing the plug 

 over "live" terminals an arc is drawn out, which is accentuated by a 

 habit of some operators of running their pencils along the grooves 

 leaving a conducting path. Such arcs caused permanent conducting 

 paths in the surface of the bakelite, despite the adoption of strenuous 

 cleaning routines. The need for a better insulating material for this 

 use became even more urgent with a demand for a test strip having 200 

 terminals Instead of 100 in the same space. 



Studies of compounds having such base materials as cellulose-nitrate, 

 shellac, hard rubber, casein, and cellulose-acetate showed the last 

 mentioned to give desirable arcing resistance. Foreign conducting 

 material on the surface was burned off by the arc; the products of 

 combustion of the small amount of cellulose acetate actually burned 

 by such an arc are largely volatile, and the residue is non-conducting. 



The compound used was found not to be sufficiently heat resistant 

 to be satisfactory for the body of the test strip. The problem was 

 solved by using it as a veneer on the test face of the bakelite strip. 



