These letters, which cover the period July-December 1877, relate to Thomas B. A. David, president of the Central District and Printing Telegraph Co. of Pittsburgh. The letters pertain primarily to a series of tests that David conducted on Edison's and Bell's telephones to determine their effectiveness. Other items deal with Edison's ongoing technical improvements of his carbon telephone. The correspondents include Edison, David, and Bell Telephone Co. officials Gardiner G. Hubbard and Thomas Sanders. Courtesy of the AT&T Archives and History Center.