AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 365

About Telecommunications
Telephone Station Equipment
Telephone station equipment is the user's gateway to the global
communications network and an array of services. While today's telephones
range from single-line telephones to multiline telephones with various features
and options, telephone station equipment can now also involve such things as
digital data terminals (for example, personal computers) or advanced
videoconferencing equipment. As sophisticated as the equipment has become,
many of the basic components and concepts are based on the original
telephones.
The first working model of a telephone was demonstrated on March 10, 1876 by
Alexander Graham Bell and his assistant, Thomas A. Watson. It 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.
Early telephones continued to be powered by direct current (dc) supplied by a
battery inside the telephone itself until, in 1894, the telephone company used a
common battery to power all the telephones connected to an exchange. The
telephone company office was called the central office (CO) , as it is today, and
this was where the battery was located.
The flow of direct current to early telephones was controlled by the receiver (or
handset ) which hung on a hook that activated electrical contacts. This hook was
called a switchhook , a term and concept that's still used today. The status of the
switchhook signals the central office about the status of the telephone station
equipment:
Idle Status. When the handset is sitting on its cradle ( on-hook ), the
switchhook contacts are open (not connected) and no current is drawn
from the CO. This signals the CO that the telephone is available to
receive calls.
Busy Status. When the handset is removed from the cradle ( off-hook ),
the switchhook contacts are closed (connected) and current flows. This
signals the CO either that the caller is requesting service or that the user
is already on a call and is not available for another call.
Likewise, 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 and that it
can stop the ringing. Originally, various bells and buzzers were used to signal
the called party to pick up the phone. But in 1878, Watson developed a bell-
ringer with a hammer attached to an armature which was, in turn, powered by
magnetic energy and operated by a hand crank. A form of this ringer is still
used in some of today's telephones.
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System Manager's Guide