Bell invented this induction balance to try to locate an assassin's bullet in the body of President James Garfield. However, the bullet was too deep for Bell's device to detect, and the President died on September 19, 1881.
The induction balance was one of the first metal detectors. Two telephone circuits near each other can produce a noise, called an induced tone. Bell's device contained two wire circuits positioned so the tone could not be heard unless the paddle passed over a metal object. Bell later created another device that was used successfully for decades to locate bullets.
