-by-Step, and
Broken-Line

PROTECTION _By K. B. Miller and S. G. McMeen_

Electrical Hazards--High Potentials--Air-Gap Arrester--Discharge
across Gaps--Types of Arrester--Vacuum Arrester--Strong
Currents--Fuses--Sneak Currents--Line Protection--Central-Office and
Subscribers' Station Protectors--City Exchange
Requirements--Electrolysis

MANUAL SWITCHBOARDS _By K. B. Miller and S. G. McMeen_

The Telephone Exchange--Subscribers', Trunk, and Toll
Lines--Districts--Switchboards--Simple Magneto
Switchboard--Operation--Commercial Types of Drops and Jacks--Manual
vs. Automatic Restoration--Switchboard Plugs and Cords--Ringing and
Listening Keys--Operator's Telephone Equipment--Circuits of Complete
Switchboard--Night-Alarm Circuits--Grounded and Metallic Circuit
Line--Cord Circuit--Switchboard Assembly

REVIEW QUESTIONS

INDEX

[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
latter was vibrated in consequence of the varying pull upon its bit of
iron, and these vibrations reproduced the sound that vibrated the
sending diaphragm.

The first public use of the electric telephone was at the Centennial
Exposition in Philadelphia in 1876. It was there tested by many
interested observers, among them Sir William Thomson, later Lord
Kelvin, the eminent Scotch a