AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 46

Background
Telephone Equipment
The first working model of a telephone consisted of a microphone (called a
transmitter ) and a small loudspeaker-like device (called a receiver ) connected
by a pair of wires and a battery.
A telephone is powered by direct current (dc) which, in early phones, was
supplied by a battery inside the phone. Beginning in 1894, COs used a
common battery to power all the telephones connected to the exchange.
The receiver for early telephones hung on a hook that activated a switch to
control the flow of direct current to the telephone. This hook was called a
switchhook, a term that is still used today. When a telephone handset is sitting
on its cradle ( on-hook ), it draws no current from the CO. When a person
removes the handset from the cradle ( off-hook ), current flows and signals the
CO that the caller is requesting service.
Similarly, the CO signals the called party by sending current to his or her phone,
causing it to ring. When the called party lifts the handset from its cradle, the
current flows, indicating to the CO that the party has answered.
Bell realized that a caller needed a way to signal the other person to pick up the
phone. After experiments with various bells and buzzers, in 1878 Bell's assistant
Watson developed a bell ringer operated by a hand crank.
When human operators handled switching, the caller used the telephone's hand
crank to ring the operator, and then told the operator the name of the person he
or she wanted to reach. If the called party was available, the operator
connected the two parties by using a cord that had plugs at each end. Each
plug had parts called a tip and a ring that functioned as conductors to complete
the electrical circuit. The operator connected the two parties by plugging in one
end of the cord into the caller's connector (called a jack ) on the switchboard,
and the other end of the cord into the called party's jack.
Once automatic switches were in place, telephone companies assigned
numbers to telephone service subscribers, and a dialing mechanism was built
into the telephone. The caller identified the called party to the switch by dialing
the called party's number.
Telephone users originally dialed numbers by using a mechanical device called
a rotary dialer . A spring wound up when turned in one direction and, on its
return to normal position, caused interruptions in the flow of current, thus
creating dial pulses recognized by the switch. The subsequent development of
the touch-tone dialer provided a further innovation: the creation of unique tones
produced by simply pressing buttons on the dialpad.
2–5
About the System