Bell is also credited with the invention of the metal detector in 1881. The device was quickly put together in an attempt to find the bullet in the body of U.S. President James Garfield. According to some accounts, the metal detector worked flawlessly in tests but did not find the assassin's bullet partly because the metal bed frame on which the President was lying disturbed the instrument, resulting in static.^[131] The president's surgeons, who were skeptical of the device, ignored Bell's requests to move the president to a bed not fitted with metal springs.^[131] Alternatively, although Bell had detected a slight sound on his first test, the bullet may have been lodged too deeply to be detected by the crude apparatus.^[131]

Bell's own detailed account, presented to the American Association for the Advancement of Science in 1882, differs in several particulars from most of the many and varied versions now in circulation, most notably by concluding that extraneous metal was not to blame for failure to locate the bullet. Perplexed by the peculiar results he had obtained during an examination of Garfield, Bell "...proceeded to the Executive Mansion the next morning...to ascertain from the surgeons whether they were perfectly sure that all metal had been removed from the neighborhood of the bed. It was then recollected that underneath the horse-hair mattress on which the President lay was another mattress composed of steel wires. Upon obtaining a duplicate, the mattress was found to consist of a sort of net of woven steel wires, with large meshes. The extent of the [area that produced a response from the detector] having been so small, as compared with the area of the bed, it seemed reasonable to conclude that the steel mattress had produced no detrimental effect." In a footnote, Bell adds that "The death of President Garfield and the subsequent post-mortem examination, however, proved that the bullet was at too great a distance from the surface to have affected our apparatus."^[132]