What breaks the tension when the mob comes to the house in Chapter 15 of "To Kill a Mockingbird"?
The mob that you refer to is made up of some men from town led by Mr. Heck Tate. They have come to see Atticus at home to talk about him defending Tom Robinson in his rape trial. When Atticus tells Heck to bring the men in, he responds by saying to Atticus that they want to speak to him outside. His answer makes it evident that the call is not a social one.
Jem, Scout, and Dill are very interested in the proceedings and watch the scene from inside the house. They hear the men discussing the fact that Tom Robinson is moving to Maycomb jail in preparation for his trial. The group is worried about the risk involved and expect trouble from a group they call "that Old Sarum bunch"—men who evidently do not want Tom Robinson to have a fair trial. The men's concerns add to the tension, and Mr. Link Deas wants to know if a change of venue cannot be requested.
The group believes that the Sarum bunch is going to do something that might bring trouble to the town, and Mr. Link Deas tells Atticus that he could lose everything. Atticus, however, is adamant that Tom Robinson will have his day in court because all of them, as he puts it, "know what the truth is." It is pertinently clear that the men want to prevent trouble at all costs. When Atticus says this, the suspense is heightened when he takes a back step and the group, at the same time, moves closer to him.
It is at this point that Jem screams out to Atticus that the telephone is ringing. At Jem's scream, the men "jumped a little and scattered, " and when Atticus calls out to Jem to answer the call, the group laughs and the strain lightens immediately.
When Jem and Atticus discuss the incident afterward, Jem compares the visiting band of men to a lynch mob. Atticus, however, assures him that the men are their friends. Their conversation and the earlier incident foreshadow the actual visit of just such a mob at the jailhouse later in the chapter when a group of men turns up to lynch Tom Robinson.
Both Jem and Scout break the tension when they come out from their hiding place at the jail where Atticus has set up watch over Tom Robinson for the night. As the men approach Atticus Scout and Jem come out from hiding and Scout sees true terror on Atticus' face when he realizes that his children are in harm’s way as well as himself. This speaks volumes about a man who would stand up to a mob and potentially give up his life but, however, when his children are involved he becomes petrified. As the children ignore Atticus order for them to leave, one of the men grabs Jem by the shirt collar prompting a kick in the shin from Scout. These events ratchet up the tension until Scout singles out one man, Mr. Cunningham, from the group and asks him how his son is doing. She unknowingly diffuses the situation by reminding Mr. Cunningham of his ties to the Finch family. Having been reminded of the true kindnesses that Atticus has done to him in the past, Mr. Cunningham orders the rest of the men to leave.