This is misleading.

The Rule is that the sum of currents on *all* the box's conductors has
to add to zero. If the box has only two terminals, the sum of the two
has to be zero -- the only way to get around that would be to put Cecil
into the box and have him suck coulombs just as fast as he can. If that
two-terminal box contains an inductor, then the current out has to equal
the current in -- that's the only way the sum of currents at the two
terminals can sum to zero. Provided, of course, that the box is very
small in terms of wavelength, and we're measuring over the long term.
It's ok to suck up and store charge for a while -- but not forever.

When you put even a third terminal on the box, you have a lot more
choices as to what you put into it -- an autotransformer, for example.
Then you can find any number of gee, whiz, Mr. Science, absolutely
wonderful things about the voltages and currents to dazzle the
technically uncertain. The four terminals of Cecil's box provide even
more opportunities to amaze. But one thing you can take to the bank,
folks: the sum of the currents on all the terminals better add to zero.
Unless, of course, Cecil is in the box.

Roy Lewallen, W7EL

Cecil Moore wrote:
Richard Harrison wrote:

Cecil, W5DXP wrote:
"Reg, you have a black box in the middle of a transmission line with a
high SWR. You measure the current into the box and out of the box, You
measure 1 amp into the box and out of the box, You measure 1 amp and 0
degrees going in and 1 amp at 180 degrees going out. This means that
both currents are flowing into the box at the same time. There is no
third wire. What`s in the box?"

A phase inverter.

You could have a center-tapped coil in the box. One end and the center
could take the input. The other end and the center could provide an
output 180-degrees out of phase with the input.


Yep, that's one answer. Another answer is a piece of low-loss transmission
line that shifts the phase by 180 degrees, i.e. 1/2WL of transmission line.

Point is that unequal currents at the input and output of a black box are
easy to achieve and do not violate Kirchhoff's laws. Although physically
small, this black box does not meet the definition of a lumped circuit.

A bugcatcher coil on a 75m mobile antenna also does not meet the
definition of a lumped circuit.