Key and the Bible
(A). Employed to discover whether plaintiff or defendant is guilty. The Bible is opened either at Ruth, chap. i., or at the 51st Psalm; and a door-key is so placed inside the Bible, that the handle projects beyond the book. The Bible, being tied with a piece of string, is then held by the fourth fingers of the accuser and defendant, who must repeat the words touched by the wards of the key. It is said, as the words are repeated, that the key will turn towards the guilty person, and the Bible fall to the ground.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894