The telephone system uses a 48-volt direct current (or 48-volt
DC) to send the voice signal on the talking circuit.
The signal to sound the telephone bell is carried on the "ringer
circuit," which uses 110-volt alternating current (or 110-volt AC).
In the 1890s, power for the talking circuit came from batteries
that were included in each telephone and in the switchboard. Later,
the "common battery" system used direct current from batteries in
the telephone office.
Early telephone systems used electrical currents travelling
through the ground as part of the circuit. This arrangement was
subject to interference from other electrical installations such as
streetcars and electric street lighting, resulting in poor sound
transmission. When metallic circuits added another wire instead of
using the ground, signal quality improved. It was well worth the
cost of the extra wire.
An electric current travelling through an open pair of wires must
have the position of the wires transposed at intervals to avoid
"crosstalk interference". This occurs when the signal travelling
through one wire injects noise into a neighbouring wire, thus
interfering with the sound quality during a telephone conversation.
Often, the rural networks operated by farmer co-operatives or
"mutuals" used less expensive equipment and materials. Barbed wire
fence lines might be used to carry the signal instead of erecting
tall poles. In the absence of other electrical currents, a ground
return was quite satisfactory. When new technology was introduced in
Alberta’s cities in the 1950s, rural systems had to be upgraded to
work effectively with the new systems.
Because the voltage in these old systems was different from the
newer 110-volt AC that was becoming a standard for most electrical
equipment, the telephone company had to transform the power from the
grid to 48 volts DC for use in its own network. This transformation
also made it generally possible to use the telephone during power
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Heritage Community Foundation and
Telephone Historical Centre All Rights Reserved