[Footnote A: For professional standing of authors, see list of Authors
and Collaborators at front of volume.]

[Illustration: OLD BRANCH-TERMINAL MULTIPLE BOARD, PARIS, FRANCE]




TELEPHONY

INTRODUCTION


The telephone was invented in 1875 by Alexander Graham Bell, a
resident of the United States, a native of Scotland, and by profession
a teacher of deaf mutes in the art of vocal speech. In that year,
Professor Bell was engaged in the experimental development of a system
of multiplex telegraphy, based on the use of rapidly varying currents.
During those experiments, he observed an iron reed to vibrate before
an electromagnet as a result of another iron reed vibrating before a
distant electromagnet connected to the nearer one by wires.

The telephone resulted from this observation with great promptness. In
the instrument first made, sound vibrated a membrane diaphragm
supporting a bit of iron near an electromagnet; a line joined this
simple device of three elements to another like it; a battery in the
line magnetized both electromagnet cores; the vibration of the iron in
the sending device caused the current in the line to undulate and to
vary the magnetism of the receiving device. The diaphragm of the