Massimo Polidoro explains Houdini’s Impossible Demonstration, which he made to Sir Arthur Conan Doyle, creator of the Sherlock Holmes stories — who, ironically, was a true believer in Spiritualism:
“Sir Arthur,” continued Houdini, “I want you to go out of the house, walk anywhere you like, as far as you like in any direction; then write a question or sentence on that piece of paper; put it back in your pocket and return to the house.”Doyle obeyed, walking three blocks and turning a corner before he wrote upon the paper. When he returned Houdini invited him to take a spoon and remove the cork ball, which had been soaking in the white ink, then to touch the ball to the left side of the slate. The ball “stuck” there, seemingly of its own volition. Slowly, it began rolling across the surface of the slate, leaving a white track as it did so. As the ball rolled, it was seen to be spelling the words: “Mene, mene, tekel upharsin,” the very same words that Doyle had written. The guests were speechless.
Houdini then explained that it was all done with trickery, not psychic powers — but Doyle never believed him.
How it was done:
Berol had been performing for years, both in Europe and America, an act in which a ball dipped in ink would spell on an isolated board the words called out by members of the audience:“Berol did this by switching a solid cork ball for one with an iron core. A magnet at the end of a rod, manipulated by an assistant concealed behind the board, caused the ball to adhere and move — apparently under its own power. After Berol retired, Houdini purchased the equipment. An assistant in the room adjacent to Houdini’s library had opened a small panel in the wall and extended the rod with a magnet through it. The ball on the slate had an iron center, of course.
“Ernst had not remembered that when Doyle returned to the room, after writing the words outdoors, Houdini had checked to make sure the slip of paper on which Doyle wrote was folded, then immediately returned it to his friend. Before doing so, the magician had switched slips. While Doyle was busy retrieving the ball from the inkwell and taking it to the board, Houdini read the words. His conversation cued his hidden assistant. Once the message had been written on the slate, Houdini asked Doyle for the folded slip to verify his words. He opened the blank paper, pretended to read from it, then switched it for the original as he returned the paper to his friend. Later, Houdini explained this switching process during his public lectures on fraudulent mediums.”