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
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.
By Gerald Mosteller
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.
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 among possibilities.
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.
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
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.
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).
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 filament.
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.
A 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 transmitter.
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 outside edge.
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, hand type;
- 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., 450 V.;
- 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., C2;
- 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 V., C8;
- One Yaxley L potentiometer, 0.1-meg., R;
- Two I.R.C. B1 carbon resistors, 0.1-meg., 1 W., R1, R6;
- 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., 1/2-W., R4;
- Two I.R.C. B1 carbon resistors, 2,250 ohms, 1 W., R5, R10;
- 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
- 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 8, 2023
(updated from original post