Why did the galvanometer needle move in opposite directions when Faraday opened and closed a circuit?

The movement of the galvanometer needle in opposite directions when Michael Faraday opened and closed the circuit is a result of electromagnetic induction.

Faraday discovered that an electric current is induced in a secondary circuit only when the magnetic field associated with the primary circuit is changing—specifically, when it is growing (as the circuit is closed) or collapsing (as the circuit is opened).1

According to Faraday’s law of induction, the magnitude of the induced electromotive force is proportional to the rate of change of the magnetic flux. Because the magnetic flux increases when the circuit is closed and decreases when it is opened, the induced current flows in opposite directions in each instance, causing the galvanometer needle to deflect accordingly. This phenomenon reflects the fundamental principle that induction is driven by the change in magnetic flux over time.1