In Chapter 15, what was Atticus' motivation for being at the jail that evening?

Atticus leaves his home to sit outside Tom Robinson's jail cell the night before the trial as a precaution. He is motivated to keep Tom safe, so that Tom will have a chance to tell the "truth" during the trial. Earlier in Chapter 15, Sheriff Tate and other community members stop by Atticus' house to discuss a change of venue for the Tom Robinson case. Mr. Link Deas tells Atticus that he's worried about the Old Sarum bunch, but Atticus dismisses this notion. Atticus mentions that tomorrow is Sunday, and the Old Sarum bunch never drinks on Sundays. Therefore, Sheriff Tate has little to worry about. One of the community members says, "This is a special occasion, though..." (Lee 194) On Sunday night, Atticus leaves his house carrying a long extension cord with a light bulb at the end. Atticus plans on looking out for Tom Robinson's welfare, just in case the Old Sarum bunch tries to harm him. Eventually, the Old Sarum bunch arrives and confronts Atticus, who is sitting in a chair, reading the newspaper with the light above him. His decision to sit outside the jailhouse could have prevented Tom Robinson from getting lynched.