Source: High school physics (Chinese)
Problem Sets:
Problem
In the circuit shown, a central primary coil is connected to a battery through a rheostat $R$. On either side sit two secondary coils, $A$ (left) and $B$ (right), each closed through a galvanometer $G$. The sliding contact of $R$ is moved to the left so that the current in the primary coil is weakened.
In both $A$ and $B$ the induced current flows in the direction that opposes the decreasing flux (Lenz's law) -- the same rotational sense as the primary current, so as to maintain the flux. The deflection lasts only while the slider is moving.
This is mutual induction. Weakening the primary current reduces the magnetic flux it sends through the secondary coils $A$ and $B$. By Lenz's law the induced current in each secondary flows so as to oppose the decrease, i.e. in the direction that tends to maintain (reinforce) the original flux through that coil. The two coils are wound alike and sit symmetrically about the primary, so their galvanometers deflect in corresponding senses. The induced currents exist only while $R$ is being adjusted; once the current is steady again they vanish.