Amherst, N.H., auction house RR Auction says the Bell letter — written to his father in 1878 after a "lightning accident" — sold Wednesday night to a Texas collector for US$92,856.
The eight-page letter contains a one-of-a-kind sketch of two connected telephones, with in-depth instructions to his family on how to ground the telephone.
Bell was responding to a letter from his parents in Brantford, Ont., telling him about how a lightning strike had damaged their wiring.
Bell's letter shows how to properly assemble and connect telephones to ensure proper function and safety.
Bobby Livingston of RR Auction says sketches by great inventors of their invention are held in the highest regard by collectors.