A Pay Phone

For years,if you were away from home and wanted to make a phone call, pay phones were your only option. Even now that most of us have phones in our pockets, pay phones aren't going anywhere. The FCC suggests having them in certain places for the sake of public safety or welfare (such as out- side a police station, where you might need to make a call after being arrested). Pay phones manage to connect calls, set prices, and accurately collect payment using only electricity drawn from the phone line. They're also basically bullet-proof, devised to withstand all manner of vandalism and theft.

In the 1980s, the telephone companies that owned the nation's phone lines required that phones draw power only while in use. When the handset is removed from the cradle, the lever releases and triggers the hook switch . That allows the phone to start drawing power, giving the user a dial tone—the signal that the phone is ready to be used. During a call, the 48 volts of electricity supplied by the phone lines power all the phone's elec- tronics, including, crucially, its master controller and LCD screen. The phone has a rechargeable NiCd battery used for only one purpose: When the handset is replaced in the cradle, the phone has to either bank the payment for the call or return coins to the caller. Each call charges the battery just enough to carry out this operation.