Seventy-Five Years of the Telephone: An Evolution in Technology

March 1876--the inventor SEVENTY-FIVE years ago--on HERE; I10,WANT transmitted byspoke and his assistant heard the first sentence to be telephone: "MR. WATSON, COME YOU." Three days earlier, U.S. Patent No. 174,465 had been granted to Alexander Graham Bell for his concept of means for making the conversion between the air vibrations of an uttered sound and their corresponding electrical undulations. On this historic occasion, Bell talked into his liquid transmitter, and Thomas A. Watson listened to a tuned-reed receiver. In this receiver, shown at the right of Fig. 1, the free end of a steel armature was caused to vibrate by the undulatory currents through an electromagnet. Bell's famous patent showed such a structure with the free end of the reed attached to the middle of a stretched membrane, as at the left of Fig. 1. In Bell's liquid transmitter, in the middle of Fig. 1, a wire attached to a sound-vibrated diaphragm varied the length of its contact with some acidulated water, and thus produced a resistance changing in accordance with the impinging sound waves. This sound-controlled variable resistance in a battery circuit provided a means of associating amplification with the conversion of speech waves into their electrical counterparts. Thus, the first telephonic transmission of information demonstrated the two general principles of making the conversion between sound and electricity which have continued to be embodied universally--after much evolution through invention, research and development --in the transmitters and receivers of commercial telephony.