A 19th-century stunt involved balancing a ruler between one's outstretched forefingers (or on the edges of butter knives), and then sliding the hands together. No matter what the starting positions (even if asymmetrical) or the speed(s) of movement, the hands will always come together at the exact center of the ruler.
A clever (and more magical) application was created in 1951 by a group of magicians who met weekly, usually in the New York apartment of Bruce Elliott. The group, known as “The Friday Night Sodality”, included Dai Vernon, Stanley Jaks, Martin Gardner, Bill Simon and Norman Jensen. Their innovation was to gimmick the ruler by inserting a hidden weight into one end, thus changing its center of gravity and producing a forced outcome away from the center. This was published as “The Ruler” in Phoenix, No. 244, Dec. 14, 1951, p. 974.
In the 1990s, the idea was reinvented (with a repositional gimmick) by Ton Onosaka, and marketed as “The Divining Rod.”