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."
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Where did Bell indicate that the bullet was too deep for his machine to register?
footnote