ACT I - SUSPICION and HYSTERIA: Everyone wonders what the girls were doing out in the woods, while the girls wonder what they can do or say to not get caught. They frantically work on a plan when they are out of the earshot of adults. The adults worry what great demonic affliction is bearing down in their midst.
ACT II - CONTEMPT: Elizabeth treats John with judgement although it seems she doesn't want to. The authorities test and question the merit of John and Elizabeth's faith, and John tests the loyalty of Mary Warren.
ACT III - GROWING SCORN: Accusations continue to fly and those who would not have questioned each other in previous circumstances now do so. The effort to produce truths grows and many characters achieve this ineffectively.
ACT IV - COMPLETE MADNESS: This crazed society is in complete control by the magistrates who have in previous acts relied on the testimony of children who reported false facts. Their effort goes to such great lengths to do what is right that they do what is wrong and we as an audience can see the absurdity of their acts, but the characters invested in the play have no idea how entrenched they have become in the lies they live.
It's difficult to identify the "tone" of each act of a play. Tone is traditionally defined as the author or speaker's attitude toward the subject under discussion. Arthur Miller wrote The Crucible as a critique of the climate of hysteria created by Senator Joseph McCarthy; thus we could argue the tone of the work overall is critical, but it's hard to make a distinction between the different acts of The Crucible (since all are equally critical).
Since the speakers of a play are the characters themselves, we could make a case that the tone of an individual act is the predominant attitude among the characters. Note that this doesn't necessarily fit traditional definitions of the literary device "tone."
Act One: The opening act's tone communicates discontent. Abby is unhappy because her affair with John has ended and she is desperate to reignite it. The town is up in arms because there is talk of witchcraft and Parris is not demonstrating clear and capable leadership. Parris complains about Abby's behavior, his fear of losing his position, his slave's rebelliousness, and his small salary. Abby spreads lies and insists that the other girls do the same, and many of the girls make false accusations against others.
Act Two: The overall tone is one of defensive. John works to defend himself against Elizabeth's anger and suspicion. Mary Warren defends her newfound influence in the court proceedings. John and Elizabeth defend their reputations to Reverend Hale when he questions them about their commitment to the church. John works to defend Elizabeth once she is arrested, and Hale defends the authority of the theocratic court.
Act Three: The tone of Act Three is one of desperate. Abby works overtime to try to make her lies plausible in and out of court and to control the other girls. Elizabeth perjures herself in a desperate act to protect John. Mary Warren turns her back on the truth and the Proctors to save her own life. Hathorne, Parris, and Danforth struggle to maintain control of the proceedings to prevent Salem from falling into chaos and rioting. Giles sacrifices himself to save others.
Act Four: The final act's tone is despairing. Innocent, pious people are executed. Elizabeth, a pregnant woman with three children, is left a widow. Giles, an elderly farmer, is tortured to death. Corruption reigns because logic and truth are ignored by an arrogant court that considers itself infallible. John Proctor, the protagonist, sacrifices his life for a moral principle.