The seven-page letter contains two drawings of the phone and instructions for running its elaborate wiring system to avoid harm from lightning strikes.
Scottish inventor Bell was responding to a letter from his parents telling him how a lightning strike had damaged their wiring between several poles.
Bell writes that he was “quite troubled” by the news and proceeds to tell them how to avoid such an incident in the future.
“If you have a good connection with a permanently moist stratum of earth, you need never fear lightning and your posts will be safe,” he says.
His drawing shows a long strand of wire running to a rectangular box, above which is written: “Bury in duck pond.”
The letter is more than a tutorial on how to ground his new invention. He opens with “My dear Papa and Mama”, tells them their new granddaughter is developing into “a healthy-fat-nice-looking baby with tremendous eyes”. He signs the letter “Your loving son, Alec” – a nickname used only by the family.
The letter is dated 10 June 1878, and was written just two years after Bell obtained the patent on the telephone and made his first call to his assistant, Thomas Watson.
Bell and his family moved to North America in 1870 and settled near Brantford, Ontario. Bell moved to Boston the following year, where he taught the deaf and later became a professor at Boston University.