2G2 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



dc line current. A wave-wound air-cored coil was used for the carrier- 

 path filter (Li in Fig. 2), and a toroid on an a.f. Permalloy dust-core ring 

 for the low-pass filter (L2 in Fig. 2). Solid wire, with Lewmex and double- 

 silk covering, was used to keep the dc resistance to a minimum. 



5.4 Capacitors 



Capacitors are divided into three categories: 



(a) Those subjected to the full hne voltage which may operate at 



up to 3 kv. 



(h) Those subjected to the amplifier high-voltage supply of 90 volts, 

 (c) Those which have negligible polarization (less than 10 volts), 



and which are often required to precise values. 



Groups (a) and (6) are of the oil-filled paper type, with, respectively, 

 four layers of 36-micron and three layers of 7-micron Kraft paper. The 

 oil is a mineral type loaded with 18 per cent resin, and the capacitors are 

 filled at 60°C and sealed at room temperature. 



Small capacitors and those of precise value as in group (c) are silvered- 

 mica capacitors. These are encased in an epoxy resin to give mechanical 

 protection and a seal against moisture. Visual inspection of all mica 

 plates is made before and after silvering, and any with cracks, inclusions, 

 stains or any other abnormality are rejected. Mica is a very variable 

 material and at times the percentage rejects were high, but probably 

 many of the reasons for rejection would not have been significant as far 

 as the life of the capacitor is concerned. However, experience has shown 



Fig. 8 — Details of housing construction. A. Armour clamp. B. Gland. C. Bulk- 

 head. D. Part of internal unit. 



