The phone line
A telephone is usually connected to the telephone exchange
by about three miles (4.83 km) of a twisted pair of No.22 (AWG)
or 0.5 mm copper wires, known by your phone company as "the
loop". Although copper is a good conductor, it does have
resistance. The resistance of No.22 AWG wire is 16.46 Ohms per
thousand feet at 77 degrees F (25 degrees C). In the United
States, wire resistance is measured in Ohms per thousand feet;
telephone companies describe loop length in kilofeet (thousands
of feet). In other parts of the world, wire resistance is
usually expressed as Ohms per kilometer.
Because telephone apparatus is generally considered to be
current driven, all phone measurements refer to current
consumption, not voltage. The length of the wire connecting the
subscriber to the telephone exchange affects the total amount of
current that can be drawn by anything attached at the
subscriber's end of the line.
In the United States, the voltage applied to the line to
drive the telephone is 48 VDC; some countries use 50 VDC. Note
that telephones are peculiar in that the signal line is also the
power supply line. The voltage is supplied by lead acid cells,
thus assuring a hum-free supply and complete independence from
the electric company, which may be especially useful during power
outages.
At the telephone exchange the DC voltage and audio signal
are separated by directing the audio signal through 2 uF
capacitors and blocking the audio from the power supply with a 5-
2
Henry choke in each line. Usually these two chokes are the coil
windings of a relay that switches your phone line at the
exchange; in the United States, this relay is known as the "A"
relay (see fig.1). The resistance of each of these chokes is 200
Ohms.