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Fig. 12.9 Box that includes a current source equal to that applied
put into the resistor from outside, the unseen current generator puts
another through it in the same direction. So the voltage drop across
the resistor, which we measure between the terminals, is twice what it
should be with 40 Ω. It would be reasonable to deduce, therefore,
that the box contained an 80 Ω resistor. If the internal source sent
current through the resistor in the reverse direction it would make
the resistance seem less than it actually was.
Here we have another example of duality: voltage in opposition
simulates higher resistance; current in opposition simulate ...