16 Jul
1998
16 Jul
'98
7:35 a.m.
When a phone is on-hook (hung-up) there is a 48V
potential on the line.
When the phone is ringing, there is 90v (nominal) AC on the line during
the ring cycle to activate the ringer. When a phone is off-hook (you're
talking) there is 12v on the line.
I think the 48V on hook and 12V off hook is historical - in the days of
long copper wires back to the exchange, it was all done from the 48V
battery - 12V was all that was left after voltage drop when the 30mA or
so (apparently the carbon microphone needs at least 23mA to work well)
had to go the several miles to your house and back. May still be like
that, come to think of it.
Those on shorter lines had higher resistance phones to compensate. In
the 1960s (? What date is the 706 anyway ?), many UK phones had the
"line drop compensator" on a plug-in module, presumably so you could
swap it for one with different resistors...
Philip.