Lucent Technologies MERLIN LEGEND System Manager's Manual page 41

MERLIN LEGEND Communications System Release 5.0
System Manager's Guide 555-650-118
2 About the System
Background
Long Distance
(Toll) Network
Figure 2–2. The Telephone Network
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
CO
CO
Customer
Premises
2
Issue 1
June 1997
Page 2-4
Customer
Premises