Bell performed a number of experiments to improve his variable-resistance telephone transmitter. In this page from his lab notes, Bell describes a test using a transmitter without an electrical circuit. He wrote, "When Mr. Watson spoke into M (the mouthpiece) I heard at E (the ear piece) the same curious mumbling half-indistinct pronunciation that had been transmitted electrically before. It is evident that the fault lies in the mouth piece. . ." This result led Bell to design the new transmitter shown in the bottom sketch.
