These two letters from February and August 1878 deal with Emile Berliner's invention of a variable-pressure transmitter, which the Bell Telephone Co. planned to use as an alternative to Edison's carbon transmitter. The correspondents are Berliner; Gardiner G. Hubbard, president of the Bell Telephone Co.; and Charles A. Cheever, manager of the Telephone Co. of New York.

Courtesy of the AT&T Archives and History Center.