AUTOMATIC SWITCHING SYSTEM 



440 



Fig. 5 — Pre-set pulse-position-dialing telephone set. 



As shown in Fig. 6, which is a schematic of the mechanism and circuit 

 of this telephone set, the handset when resting in its supporting cradle 

 depresses the switchhook pins and causes two bell cranks to operate two 

 sets of switchhook contact assemblies. One of these contact assemblies 

 is controlled solely by the position of the handset while the other con- 

 tacts are controlled jointly by the handset and by a magnetic locking 

 device. This magnetic locking device consists of a permanent magnet 

 yoke wliich holds the contacts in the position shown after the removal 

 of the handset from its cradle until direct current of the correct polarity 

 is allowed to flow in the windings of a latch magnet. 



These two sets of switchhook contacts jointly control the connection 

 of any of three subdivisions of the apparatus in the telephone set to the 

 line to the central office. If the handset is removed from its cradle to 

 originate a call, the free set of switchhook contacts releases to complete 

 a circuit through the latched set of contacts to the signaling equipment 

 of the station. In this signaling condition the voice transmission ecjuip- 

 ment remains disconnected from the circuit; thus, interference and 

 transmission losses caused by voice transmission equipment are avoided 

 during signaling. Upon completion of signaling direct current is pro- 

 vided from the central office to trip the latched switchhook contacts. 



