I would say that Atticus's control is mostly positive, but that the jury's effect is almost entirely negative. Prosecutor Horace Gilmer also plays a major part in the trial, especially in his cross-examination of Tom. Atticus does his best to bring out the facts of the altercation between Tom and Mayella, and he treats all of the witnesses--Sheriff Tate, Bob, Mayella and Tom--with respect, unlike Gilmer's insulting questioning of Tom and his repeated references to him as "boy." Atticus ascertains that Mayella never receives medical treatment or an examination by a physician; that Mayella is bruised on the right side of her face and around her neck; and that Bob is left-handed. Atticus treats Mayella with kid gloves at first, and his simple questions bring responses that contradict her other answers. In the end, Atticus is forced to get tough, and he "rained questions on her."
Somehow, Atticus had hit her hard in a way that was not clear to me, but it gave him no pleasure to do so. (Chapter 18)
Atticus presents Tom as a "quiet, respectable, humble Negro who had the unmitigated temerity to 'feel sorry' for a white woman," but this has little effect on the jury. When Tom made the admission that he had felt sorry for Mayella, "the damage was done." Atticus's emotional summation declares Tom's innocence, blames Mayella for having "tempted a Negro" and only suggests that Bob may have been involved. He allows the jury's conscience to decide Tom's fate, asking that they "do your duty."
But the jury's only surprise is that it took them several hours to come to a guilty verdict. Atticus knew beforehand that "the jury couldn't possibly be expected to take Tom's word over that of the Ewells'," and the most he could expect was to "jar the jury a bit." In the end, the all-white jury came to the conclusion that Atticus expected, and Scout saw the truth before it was announced.
A jury never looks at a defendant it has convicted, and when this jury came in, not one of them looked at Tom Robinson. (Chapter 21)
Atticus had managed to take the town a "baby-step" closer to racial fairness, but
... in the secret courts of men's hearts, Atticus had no case. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed. (Chapter 25)