How does Harper Lee create and sustain tension in chapter 18 and 19?

In chapter 18, the trial continues. We see Mayella herself take the stand. Lee makes us feel sorry for her, as she is nothing but a frightened and uneducated "child" (although she is over 18, she is extremely immature socially). Then, Atticus builds his own case - the case of a victim who was not raped but beaten by her own father. He essentially breaks down the prosecutions case, clearing up the significance of the fact that she was injured on the right side of her body - something Tom Robinson could not have done. She breaks down, but does not change her story at all. Instead, she begins a tirade in the courtroom and the prosecution rests leaving Atticus to call his only witness, Tom himself, to the stand.

The build-up in these two chapters is that of a standard courtroom drama. The reader gets to hear the evidence presented just as they would in an actual court of law. Like the jury sitting in the stands, or like Jem and Scout in the balcony, we see each aspect of the night in question revealed to us. As the truth becomes clearer to us, we should begin to side with the defense (if we have not already begun to do so) even though we know that the outcome, given the social situation and racial tensions of the era, will likely not be in Tom's favor.