Manuscripts/Mixed Material Script by Thomas A. Watson, December 21, 1914
B. It had never entered my head to do so. I feel sure I should have perfected it if the telephone had not absorbed almost all my attention after the discovery of June 2nd, 1875. As I remember it, this is how it happened. My harmonic telegraph as you know depended on the law of sympathetic vibrations. An intermittent current of a given musical pitch was produced by each transmitter to which the receiver would respond that had its spring tuned to that note. You remember how carefully those springs had to be tuned to prevent conflict in the telegraph signals.
Watson. Keeping those springs in tune was one of your constant troubles. When you were tuning a spring you would press it against your ear which enabled you readily to compare its pitch with the pitch of the transmitter vibrating in the other room.
Bell. I was constantly listening to electromagnets in that way to study the sounds produced in them by intermittent currents. On that famous afternoon of June 2nd, 1875, I had arranged in one of the rooms in Williams' attic three transmitters of three different pitches, three receivers with springs tuned to the same pitches, three signaling keys and a galvanic battery. In the other room were duplicates of these receivers. All these were connected on a single wire running between the two rooms. That afternoon you were in the room with the receivers, I was in the room with the transmitters, receivers and keys, getting ready to send you some Morse signals when suddenly I observed one of the receiver springs