As was mentioned in the previous post, Jem's full name is given in Chapter One. After Scout and Jem meet Dill for the first time, Jem jokes about Dill's full name, which is Charles Baker Harris. Jem finds it amusing that such a long name belongs to such a short child. Dill responds by saying, " 's not any funnier'n yours. Aunt Rachel says your name is Jeremy Atticus Finch" (Lee, 6). In the Grand Central Publishing edition of the novel, Jem's first name can be found on page 8. Also, Mrs. Dubose calls Jem by his full name when he is walking with Scout past her home in Chapter 11. Mrs. Dubose continually ridicules the Finch children throughout the novel. Mrs. Dubose yells, "Jeremy Finch, Maudie Atkinson told me you broke down her scuppernong arbor this morning" (Lee, 64). Jem is rarely called by his full name throughout the novel, partially because the story is narrated by Scout.
In chapter 1, Scout gives an account of the first Finch family member to settle in Maycomb County, how her father became a lawyer, and information about her parents' life together before her mother died. She also recounts the day they first meet Charles Baker Harris, or Dill. Jem seems to make fun of Dill's full name and Dill shoots back, "''s not any funnier'n yours. Aunt Rachel says your name's Jeremy Atticus Finch'" (7).
Another time Jem's first full name is used is in chapter 8, after Miss Maudie's house fire is put out, and the family goes back home to have some hot chocolate. Atticus asks the children where they got that mysterious blanket around Scout's shoulders and they deduce that it must have been Boo Radley who gave it to her. Jem gets very excited and tells Scout that if she had just turned away from watching the fire for a minute, she could have seen Boo Radley, the mysterious shut-in on their block. Atticus sternly advises Jem the following: "Do not let this inspire you to further glory, Jeremy" (72).
The name is found on pages 7 and 72 if you are using the Perma-Bound, Time Warner Books edition. Otherwise, search for it in chapters 1 and 8.
Jem's full name is Jeremy Atticus Finch. The page number on which it's found will vary with the edition of the book which you are reading.