[Illustration: This model of Bell’s first telephone is a duplicate
of the instrument through which speech sounds were first transmitted
electronically, 1875.]
The telephone we use today is very different in design from the first
instrument invented by Alexander Graham Bell, but it works on the same
principle. As soon as Bell proved his invention practical, he foresaw it
could link homes with offices, sweep aside the isolation of farms, and
bind together cities and nations with electrically transmitted speech.
Alexander Graham Bell had prepared himself to follow the professional
footsteps of his father and grandfather in the teaching of proper
articulation and the correction of speech defects. He became a teacher
of speech to the deaf. Early in his training, his investigations into
the nature of sound led him to study electricity. It was out of this
work, together with his understanding of the organs of speech and
hearing, that his invention grew.
He attempted to apply sound to telegraphy in a device called the
harmonic telegraph. He hoped it would transmit several Morse messages
tuned at differing levels over the same circuit simultaneously. While he
was working with this device, Bell conceived the principle of the
telephone.
He told his young mechanical assistant, Thomas A. Watson: “If I can get
a mechanism which will make a current of electricity vary in intensity
as the air varies in density when a sound is passing through it, I can
telegraph any sound, even the sound of speech.”