January 1939 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Modulating a light beam for secure communications was not a
new concept is 1939 when Gerald Mosteller invented his device,
but doing so with inexpensive equipment, using 'outside-the-box'
thinking, was new. Exploiting the relatively recently discovered
physical phenomenon of 'skin effect,' his system used a specific
range of frequencies to modulate the filament of a standard
flashlight type incandescent light bulb that could effect temperature
changes - and therefore intensity changes - rapidly and of significant
amplitude to transmit information in the audio frequency range.
Mr. Mosteller's contraption evolved as the result of a
college thesis project. There does not exist a plethora of modern-day
modulated light communications systems using incandescent bulbs
as the source, so it is safe to assume insurmountable physical
and/or financial obstacles prevented it from going mainstream.
There are, of course, many modulated light communication devices
in use, but primarily at infrared and ultraviolet wavelengths.
The Skin Effect Talking Lightbeam
The University of Southern California has cooperated in making
available to Radio-Craft readers this first detailed description
of the principle and equipment comprising the "skin-effect"
lightbeam telephone discovery of a U. of S. C. graduate. Anyone.
can easily duplicate Mr. Mosteller's experiment.
Gerald Mosteller, University of Southern California graduate,
is shown with a portion of his apparatus for sending sound over
a lightbeam, using an ordinary 5-cent flashlight bulb and simplified
radio parts. His discovery formed the thesis for his Master's
degree at the University.
A new means of transmitting sound over a beam of light by
use of amplifiers and an ordinary 5-cent flashlight bulb constituted
the writer's thesis, for his Master's degree at the University
of Southern California last June.
This simplified form of sending music, voice and other sounds
over a lightbeam, discovered during studies in Physics, has
been the subject of experiments by scientific laboratories over
the country with expensive equipment for a period of years.
Its use in secret communication by the army and navy, landing
of airplanes with lights that penetrate fog, and adaptability
by automobile races in communicating with their pits, are given
In contrast to radio communications, messages cannot be intercepted
except by instruments set up directly in the beam of light.
By using infra-red filters an invisible beam can be created
which further prevents interception.
The receiver consists of a photoelectric
cell "detector" followed by 3 stages of A.F. amplification.
Condenser C1 and resistors R3 and R4 are mounted directly behind
the P.E. cell in the reflector.
The transmitter consists of a speech amplifier,
oscillator and ordinary flashlight bulb. The A.F.-modulated
current flows along the surface of the filament (skin-effect)
causing rapid fluctuations of the light intensity (which, in
turn, are translated into intelligibility by the receiver).
How it Works
By means of a radio-frequency oscillator and equipment similar
to that of the ordinary home radio set, sound is amplified and
the output caused to modulate the oscillator.
A "skin effect" is created on the surface of the light filament
and instead of heating the entire filament as heretofore, the
current travels along the surface . Thus the filament may change
temperature fast enough to transmit sound.
Skin effect is a phenomena of alternating currents. The maximum
inductive reactance in a wire or filament is at its axis. In
general the inductance offered to other filaments of current
decreases as the distance from the axis increases. Thus the
current is distributed so that it is greatest near the surface.
The heat generated by this current is proportional to the square
of the current. The heat is radiated from the surface, conducted
to the cooler center and used in raising the temperature of
The high-frequency alternating current (radio-frequency)
causes the skin effect, the modulation of this current varies
the surface temperature of the filament so that the light varies
with the modulation. The conductivity of the metal carries the
surface heat toward the center of the filament when the modulated
current is great, thus allowing the surface to cool more rapidly
so the radiation may follow the modulated current down. This
improves the audio response at high frequencies.
curve was drawn to show the advantage due to skin effect. It
was shown as a percentage of the oscilloscope readings of a
sine wave at some frequency compared with the reading at 768
cycles per second. This curve of the efficiency of lightbeam
modulation was measured on an oscilloscope having its vertical
sweep connected to the output of the receiver. The input to
the oscilloscope was a beat-note oscillator connected to the
A photoelectric cell is mounted in the focus of a parabolic
reflector which may be made from an electric heater by removing
the heating element. The particular heater purchased was selected
after testing several because it concentrated rays from a distant
light at a very small area. It is necessary that the rays focused
by the parabola form a sharp-pointed cone of light which can
get into the curved plate of the cell rather than hit on its
Directly behind the cell is mounted a midget 0.01-mf. coupling
condenser, C1; a 1-meg. resistor, R3, through which the cell
receives its positive voltage and the 5-meg. resistor, R4, which
is the grid resistor for the 1st tube. The shielding afforded
by the metal reflector and the wire shield in front of it is
enough for shielding these elements, however they might be further
shielded to cut down A.C. hum by wrapping them in tinfoil and
grounding the foil.
Similar phenomena have been developed before by means of
high-voltage equipment and the use of cathode-ray and neon lights,
but never before with simple equipment including the use of
a "nickel" (5c) battery lamp.
The 12-meter grid coil is wound with 18 turns of No. 14 enamel-covered
wire on an air core 2 x 1/2-in. inside dia. The 12-meter plate
coil has 12 turns of E.C. wire on a 1 1/2 x 3/4-in. dia. air
core; the coupling to the Mazda-lamp circuit is accomplished
by means of a single turn of No. 14 E.C. wire around the center
of the plate coil.
One position of the S.P.D.T. switch connects the mike through
a 4 1/2 V. mike battery, the other cuts the battery out so a
low-impedance phono pickup may be used. The single-button carbon
mike used had low impedance.
List of Parts
- One American type SB single-button carbon microphone,
- One Yaxley No. 75 phone plug;
- One Yaxley No.1 phone jack;
- One H&H S.P.D.T. toggle switch;
- One u:r.c. No. CS6 microphone input transformer;
- One Aerovox No. PR25 condenser, 10 mf., 25 V.;
- One Cornell-Dubilier electrolytic condenser, 4 mf.,
- One Yaxley No. M25MP volume control potentiometer, 0.25-meg.;
- One I.R.C. type B1 carbon resistor, 1,250 ohms, 1 W.;
- One I.R.C. type B1 carbon resistor, 1,500 ohms, 1 W.;
- One U.T.C. No. C823 class B input transformer, 6A6 plates
to two 6A6 grids;
- One U.T.C. No. CSR modulation transformer, 6A6 plates
to 30,000 ohms;
- One Ohmite "Red Devil" resistor, 400 ohms, 10 W.;
- One Aerovox type 484 tubular condenser, 0.001-mf.;
- One National type R100 high-frequency R.F. choke;
- One Cardwell "Midway" type MR50BD split-stator condenser,
50 mmf, both sections;
- Three Sylvania type 6A6G tubes;
- One Eveready "Mazda" spotlight reflector and light bulb;
- One chassis and hardware;
- One power supply, 450 V., 100 ma.;
- Two 12-meter coils (see text).
- One RCA PE. cell, No. 921;
- One Aerovox No. 450 condenser, 0.01-mf., C1;
- One Aerovox type 484 paper condenser, 1 mf., 400 V.,
- Two Aerovox type PB25 electrolytic condensers, 25 mf.,
25 V., C3, C5;
- One Cornell-Dubilier electrolytic condenser, 8 mf.,
200 V., C4;
- Two Aerovox type 484 tubular condensers, 0.1-mf., 400
V., C6, C9;
- One Aerovox type 484 tubular condenser, 0.003-mf., 400
- One Yaxley L potentiometer, 0.1-meg., R;
- Two I.R.C. B1 carbon resistors, 0.1-meg., 1 W., R1,
- One I.R.C. B1 carbon resistor, 75,000 ohms, 1 W., R2;
- One I.R.C. B1/2 carbon resistor, 1 meg., 1/2-W., R3;
- One I.R.C. B1/2 carbon resistor, 5 megs., 1/2W., R4;
- Two I.R.C. B1 carbon resistors, 2,250 ohms, 1 W., R5,
- One I.R.C. B1 carbon resistor, 0.25-meg., 1 W., R7;
- One I.R.C. B1 carbon resistor, 20,000 ohms, 1 W., R8;
- Two Yaxley type M potentiometers, 0.25-meg., R9 (volume
control), R12 (tone control);
- One I.R.C. B1 carbon resistor, 50,000 ohms, 1 W., R11;
- One ohmite "Red Devil" resistor, 200 ohms, 10 W., R13;
- Two Sylvania type 6J7 tubes;
- Two Sylvania type 6F6 tubes;
- One Thordarson tapped input choke, No. T7431;
- One Wright DeCoster permanent-magnet magnetic loudspeaker,
with output transformer to 6F6's in push-push class B; One
power supply, 250 V., 80 ma.
Posted August 13, 2015