Page "Telephone" Paragraph 16

The phone's ringer ( A7 ) is connected to the line through a capacitor ( A6 ), a device which blocks direct current but passes alternating current.

So, the phone draws no current when it is on hook ( a DC voltage is continually connected to the line ), but exchange circuitry ( D2 ) can send an AC voltage down the line to ring for an incoming call.

( When there is no exchange, telephones often have hand-cranked magnetos to make the ringing voltage.

) When a landline phone is inactive or " on hook ", the circuitry at the telephone exchange detects the absence of direct current and therefore " knows " that the phone is on hook ( therefore, only AC current will go through ) with only the alerting device electrically connected to the line.

When a party initiates a call to this line, the exchange sends the ringing signal.

When the called party picks up the handset, they actuate a double-circuit switchhook ( not shown ) which simultaneously disconnects the alerting device and connects the audio circuitry to the line.

This, in turn, draws direct current through the line, confirming that the called phone is now active.

The exchange circuitry turns off the ring signal, and both phones are now active and connected through the exchange.

The parties may now converse as long as both phones remain off hook.

When a party " hangs up ", placing the handset back on the cradle or hook, direct current ceases in that line, signaling the exchange to disconnect the call.