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MERLIN LEGEND Communications System Release 6.0
System Manager’s Guide
555-660-118
Issue 1
February 1998
About the System
Page 2-4Background
2
Figure 2–2. The Telephone Network
Telephone Equipment 2
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.
CO
Customer
Premises
CO
Customer
Premises
Long Distance
(Toll) Network