AUTOMATIC SWITCHING SYSTEM 447 



i.s usctl at many points throughout the .switching network, the screen- 

 grid pentode in the patli selection pi'ocessc^s in tlie switching network, 

 and the octode tor miscenan(M)us i)ui"poses in the hn(\ trunk, niniilxM' 

 group and other circuits. 



The dry-reed switch, which is usinl as the contact element in many fast, 

 rehiys as well as in the metallic talking path through the oflice, is shown 

 in Fig. 3. This switch consists of two permalloy I'o Is sealed in opjjosite 

 ends of a small glass tube which is filled with an inei't gas. The o\'er- 

 lapping ends of the nxls normally ha\'e a gap l)etw(>en them and the 

 ai)i)lication of a magnetic field coaxial with the rcH'ds will cause them to 

 pull togethei' and close a metallic path from one rod or reed to the other 

 through I'hodium plating at the contacting ends. The dry-reed switch 

 has an extremely small operate and I'elease time, and because of the gas 

 sealed and permanently adjusted construction prox'ides a highly reliable 

 dirt -free contact for low current applications. The dry-reed switch and 

 relays employing it were developed by W. 13. EUwood. Mercury contact 

 relays, also of a sealed and permanently adjusted construction, are used 

 where fast operation at heavier currents is required. A sectional drawing 



Fig. 3 — Glass-sealed dry-reed switch. 



of a mercury contact relay is shown in Fig. 4. These relays were developed 

 by J. T. L. Brown and C. E. Pollard. Dry-reed relays and mercury relays 

 are described in Electrical Engineering, Vol. 66, pp. 1104-1109, Novem- 

 ber, 1947, and in Bell System. Monograph, 1516. 



THE PRE-SET SUBSCRIBER'S TELEPHONE 



111 order to eliminate direct control of any common equipment by 

 the subscriber and thereby to reduce the holding time of the dialed in- 

 formation receiving circuits and the associated subscriber-connecting 

 circuits, the experimental pre-set dial telephone set shown in Fig. 5 was 

 designed for this system by K. S. Dunlap, H. E. Hill and D. B. Parkin- 

 son. Eight selector finger wheels are grouped on a common shaft with 

 only their edges visible across the front of the telephone housing. Each 

 finger wheel is provided with ten indentations along its exposed peri- 

 phery. Each indentation is designated by an engraved number or group 

 of letters conforming to the telephone directory numbering system and 

 each indentation is of suitable configuration to permit a subscriber's 



