In the circuit shown in Fig, is a pure resistor,
is an inductor of negligible resistance (as compared to
),
is a
ac source of negligible resistance. With either key
alone or
alone closed, the current is
. If the source is changed to
the current with
alone closed and with
alone closed will be respectively
Current remains unchanged in . However, it becomes half in
, because reactance is doubled on doubling the frequency