= Solution
The coils form a <mutual induction> apparatus. Changing primary <electric current> changes secondary <magnetic flux linkage> $\Lambda_2$. <Faraday's law of induction> gives
$$
\boxed{\mathcal E_2=-\frac{d\Lambda_2}{dt}.}
$$
On closing the primary circuit, increasing <electric current> produces a transient secondary <electric current> and <galvanometer> deflection. By <Lenz's law> its direction opposes the increasing <magnetic flux>. Once primary <electric current> and core <magnetization> are steady, the derivative vanishes: the <galvanometer> returns to zero despite the continuing <magnetic field>.
On opening the circuit, the decreasing <magnetic flux> induces a secondary <electric current> tending to sustain that flux. The <galvanometer> deflects oppositely, then returns to zero. \b[Making and breaking produce opposite transient deflections; steady primary current produces no sustained induced current.]
For fixed linear coils, $\Lambda_2=MI_1$ and $\mathcal E_2=-M\,dI_1/dt$, with <mutual inductance> $M$. Opposite equal changes of primary <electric current> give opposite equal areas under the induced-<electromotive force> curves in that model. Peak sizes can differ with switching times. Real iron-core <magnetic hysteresis> and remanent <magnetization> can also change the flux excursion, so equality of the actual peaks is not required.
Back to article page
