368 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1957 



be employed because of the low voltage protection provided for the tran- 

 sistorized circuits. Both cost and space savings an; realized in these ca- 

 pacitors since no cans or potting are rerjuired due to the stability of the 

 Mylar dielectric under moisture exposure. 



Another new type of capacitor has foinid widesprc^ad use in the PI 

 system. This is a solid tantalum electrolytic capacitor used in place of 

 the usual paste or liquid electrolytic capacitor. The solid electrolyte is 

 manganese dioxide deposited upon the capacitor surfaces. The anode is 

 made from tantalum metal and upon its surfaces is deposited the tanta- 

 lum oxide which forms the dielectric. The cathode is an enveloping metal 

 completing the capacitor structure. This new design of capacitor is now 

 available in values up to 100 microfarads in a very small volume. It is 

 expected to be less expensive than other electrolytic capacitors while at 

 the same time providing a rugged structure which is relatively inert elec- 

 trochemically and which has better stability in operation and storage. 

 Fig. 12 shows prototype models of typical solid tantalum units. 



4.4 Transformers 



Transformer needs in the PI system are met by two miniature struc- 

 tures wliich were made possible by the use of low power transistor cir- 

 cuits. The carrier frequency units employ a manganese zinc ferrite core, a 

 spool winding and wire terminals which permit assembly on printed 



Fig. 13 — Carrier and voice frequency transformers for PI carrier. 



