“Notes on a Strange World: Houdini’s Impossible Demonstration”, 2006-07 (; backlinks; similar):
[Account of a magic trick demonstrated by Harry Houdini to Sir Arthur Conan Doyle (as related by Houdini’s lawyer in his memoirs), which Houdini intended to caution Doyle in his enthusiasm for seances/mediums/paranormal by showing how Doyle could be fooled. Doyle was fooled, but apparently did not believe Houdini’s assurance that it was merely a trick as Houdini did not disclose how he did it.
The trick involved a black slate writing board suspended in the middle of a room by wires while a cork ball soaked in white ink, one of several Doyle cut open to prove there was no magnets or electronics involved; Doyle went outside and wrote down a phrase (“Mene Mene Tekel Upharsin”) on a piece of paper; on returning, Doyle showed the phrase to Houdini, and then Doyle placed the cork ball against the slate; the ball did not fall but stuck to the slate, and, moving on its own, wrote out the phrase.
The trick was never disclosed, but was almost certainly based on a trick Houdini bought from a fellow magician & friend, Max Berol. In this trick, the magician reads the paper slip and secretly signals it to an assistant, possibly steganographically via pre-arranged gestures or choices of words in their patter; the authentic cork ball is swapped for a magnetic one via sleight-of-hand; it is then held against the slate board by a long thin rod which moves solely in the space ‘behind’ the board where the subject cannot see, by the assistant who is hidden behind a wall and manipulating the rod through a small hatch; the assistant, having been told the phrase by the magician’s encoded message, can now write the phrase backwards on the board by carefully moving the rod.]