Why did Faraday’s galvanometer needle move only when the primary circuit was switched on or off?

The galvanometer needle moved only when the primary circuit was switched on or off because electromagnetic induction depends on a changing magnetic field or magnetic flux, rather than a steady one.12

When Michael Faraday closed or opened the primary circuit, the magnetic field around the electromagnet would grow or collapse, respectively. According to Faraday’s law of induction, the magnitude of the electromotive force induced in a secondary circuit is proportional to the rate of change of this magnetic flux over time.1

Because a steady current produces a constant magnetic field, there is no change in flux and thus no induced current to deflect the galvanometer needle. The needle only jumps during the moments of transition—when the current is being established or interrupted—because those are the only times the magnetic flux is actively changing.13