Why does Atticus ask Bob Ewell to write his name while on the stand in "to kill a mockingbird"

While Bob Ewell is on the witness stand, Atticus asks him to write his name on a piece of paper. The reasoning behind Atticus' request is to show the jury that Bob Ewell is left-handed ... The reasoning behind Atticus' request is to show the jury that Bob Ewell is left-handed