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BELL SYSTEM TECHNICAL JOURNAL 



contacts cause an arc to flash across the surface of the phenol fiber. 

 This is a condition peculiar to telephone apparatus and a special test 

 has been designed which simulates service conditions. An insulator 

 cam (see Figs. 7 and 8) is prepared as the test specimen. The cams 

 are rotated at a speed of 10.5 to 11.5 revolutions per minute. Two 

 metal cams are attached concentrically with each surface of the phenol 

 fiber cam. Attached to a brush that wipes over the cam tensioned to 



BLANK OUT BLACK PORTIONS 



COPPER SHEET 

 (0.0201" thick) 



ARCING CAM 

 Fig. 7 — Arc resistance test cam. 



a pressure of 60 grams is a circuit containing S}/2 counting relays which 

 supplies a severe inductive load. This is representative of a severe 

 service condition. Failure is indicated when tracking of carbonaceous 

 material shorts a cam segment the distance of 15° or when the 1/32" 

 thick material is punctured, and the test is then stopped automatically. 

 A good grade of fiber will resist over 1,800 revolutions whereas a poor 

 grade will fail in 4 to 100 revolutions. 



