This type of bridge circuit has several implementations (built from transformers, resistances, or operational amplifiers, etc.). The operation of the circuit has been introduced here by a single transformer hybrid used in analogue telephone.
The primary and secondary windings of the transformer with ratio of 1:1 consist of two half-windings (L1-L2 and L3-L4), respectivelly.
The I current of the microphone forks in node 3. The current in the two branches will be the same if the impedance of the line and the
impedance of the BN (Balanced Network) is the same. In this case the voltage inducated by current I between points 4-4' will be zero,
i.e. the signal of the microphone will bypass the speaker.
The power of the signal from the microphone will be splitted between the line and the BN (this half will be dissipated). In this way the
attenuation between the neighboring points of hybrid is 3 decibells.
BN tries to balance hybrid coil (the bridge), but the success is limited, because cable (line) corresponds to the distributed element modell,
while BN has been constructed from lumped components.
It follows that the attenuation between the opposite ports (the closing attenuation) is not infinite but it will have a finite (and measurable) value.