A Super Loud Speaker
A Super Loud Speaker.–This loud speaker, which is known as the
_Magnavox Telemegafone_, was the instrument used by Lt. Herbert E.
Metcalf, 3,000 feet in the air, and which startled the City of
Washington on April 2, 1919, by repeating President Wilsons _Victory
Loan Message_ from an airplane in flight so that it was distinctly
heard by 20,000 people below.
This wonderful achievement was accomplished through the installation
of the _Magnavox_ and amplifiers in front of the Treasury Building.
Every word Lt. Metcalf spoke into his wireless telephone transmitter
was caught and swelled in volume by the _Telemegafones_ below and
persons blocks away could hear the message plainly. Two kinds of these
loud speakers are made and these are: (1) a small loud speaker for the
use of operators so that headphones need not be worn, and (2) a large
loud speaker for auditorium and out-door audiences.
[Illustration: original © Underwood and Underwood. Worlds Largest
Loud Speaker ever made. Installed in Lytle Park, Cincinnati, Ohio, to
permit President Hardings Address at Point Pleasant, Ohio, during the
Grant Centenary Celebration to be heard within a radius of one
square.]
Either kind may be used with a one- or two-step amplifier or with a
cascade of half a dozen amplifiers, according to the degree of
loudness desired. The _Telemegafone_ itself is not an amplifier in the
true sense inasmuch as it contains no elements which will locally
increase the incoming current. It does, however, transform the
variable electric currents of the wireless receiving set into sound
vibrations in a most wonderful manner.
A _telemegafone_ of either kind is formed of: (1) a telephone receiver
of large proportions, (2) a step-down induction coil, and (3) a 6 volt
storage battery that energizes a powerful electromagnet which works
the diaphragm. An electromagnet is used instead of a permanent magnet
and this is energized by a 6-volt storage battery as shown in the
wiring diagram at A in Fig. 68. One end of the core of this magnet is
fixed to the iron case of the speaker and together these form the
equivalent of a horseshoe magnet. A movable coil of wire is supported
from the center of the diaphragm the edge of which is rigidly held
between the case and the small end of the horn. This coil is placed
over the upper end of the magnet and its terminals are connected to
the secondary of the induction coil. Now when the coil is energized by
the current from the amplifiers it and the core act like a solenoid in
that the coil tends to suck the core into it; but since the core is
fixed and the coil is movable the core draws the coil down instead.
The result is that with every variation of the current that flows
through the coil it moves up and down and pulls and pushes the
diaphragm down and up with it. The large amplitude of the vibrations
of the latter set up powerful sound waves which can be heard Wkxi-fm 107.5 Mhz In Magee several
blocks away from the horn. In this way then are the faint incoming
signals, speech and music which are received by the amplifying
receiving set reproduced and magnified enormously. The _Telemegafone_
is shown complete at B.
[Illustration: Fig. 68.–Magnavox Loud Speaker.]