At the end of Act 1, the Inspector returns. Why might Priestley have decided to end the first act this way?

The level of tension in the play starts extremely low, builds gradually as the Inspector enters, and builds more as the characters come to understand the fate of Eva and their roles.

Indeed, at the end of Act One, structured by Priestley so as to end on a point of tension, we discover that it is not just Birling and Sheila, but also Gerald, who is involved in Eva Smith’s demise. The comfortable, warm atmosphere of the opening has been largely destroyed by the time the curtain comes down at the end of the act,