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CONTENTS xi

CHAPTER VIII
PAGE
ELECTRICAL MEASURING INSTRUMENTS . . . . . . . . 116
'Meters — A Simple Voltmeter and Ammeter — A Portable Volt-
meter and Ammeter — Calibrating the Meters — Connecting the
Meters — Galvanoscopes and Galvanometers — Simple Galvanoscopes
— Astatic Galvanoscope — Astatic Galvanometer — How to Make a
Wheatstone Bridge — Resistance-Coils — How to Use a Wheatstone
Bridge for Measuring Resistance.

CHAPTER IX

BELLS, ALARMS, AND ANNUNCIATORS . .

How tO Build an Electric Bell — An Electric Alarm — An Annun-
ciator — Push-Buttons — Bell Systems — A Burglar Alarm.

140

CHAPTER X

ELECTRICTELEGRAPHS............
The First Telegraph — The Principle Of the Telegraph — The Key
—The Sounder— How to Make a Simple Key and Sounder— Con-
necting the Instruments —. A Complete Telegraph Set — How to
Build a Telegraph Relay — Connecting a Relay — How tO Learn to
Telegraph — The Alphabet — Operating.

150

CHAPTER XI

MICROPHONES AND TELEPHONES . . . . . . . . . . I70
Microphones — How to Hear a F ly’s F ootstep — Telephones —
The Principle Of the Telephone — The Telephone Transmitter — The
Telephone Receiver — How to Build a Telephone — Telephone R3-
ceivers — A Home-made Telephone Receiver — A Home-made Tele-
phone Transmitter — A Complete Telephone Instrument — A Desk-

Stand Type Of Telephone — A Telephone Induction Coil — Connect-
ing the Telephones.

CHAPTER XII

INDUCTION COILS . . . . . . . . . o. . . . 194
A Medical Coil or Shocking Coil — Spark Coils — The Principle of
the Spark Coil — Building a Spark Coil — The Core — The Primary

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