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coupling. The question at once arises: with how much? The shape of
the curve is different for every value of x. Since the primary purpose
of the coupling is to transfer the signal from one circuit to the other,
a natural coupling to try for a start is the amount that transfers it
most effectively. This is not, as might at first be supposed, the
greatest possible coupling. It is actually (with low-loss circuits) quite
small, being simply χ = r.
For this value, called the critical coupling, the curve is Fig. 20.76.
The curve applies to a pair in which the two circuits have the same
L/r;
if there is any difference, an approximation can be