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Hicken65
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This is a Bell System 3-slot rotary dial payphone arranged for prepay service. This means it would be configured for ground start to get dial tone. The phone is connected by two wires (tip & ring) to the central office (CO). The ring wire is connected to one side of a line relay winding at the CO. The other side is of the winding is connected to negative 48-volt battery. So a ground on the ring wire at the payphone will operate the line relay. An operated line relay initiates a series of events at the CO to supply dial tone to the line. The payphone housing is always grounded to protect users from foreign voltages. Both the receiver and transmitter contacts in the handset are connected through the payphone’s internal wiring to the ring wire. So touching the grounded housing with one end of the pull-tab and the other end to the exposed transmitter contact would apply ground to the ring wire, and dial tone would be heard in the receiver. Ground is normally supplied through a pair of contacts that are closed when the hopper trigger is deflected by depositing a coin. The deflected hopper trigger also removes a short circuit across the dial contacts. Since no coin was deposited, the short across the dial remains and dialing would be ineffective. So, in this case, David would get dial tone but he wouldn’t be able to dial out.
posted 5 years ago in WarGames (1983)