March 1930 Radio News
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Some of the earliest television display schemes were mechanically
scanned light projection systems rather than electronically raster
scanned cathode ray tubes. This 1930 vintage article from Radio
News magazine reports on a scheme developed by Arthur Watson
whereby a specially formed rotating
disk served as the rotating reflecting surface to produce the light
scanning action. This invention was hailed as a breakthrough that
would finally make commercial TV available to the masses. "Mechanical
televisions" worked by transmitting scanned images of the original
subject in the form of amplitude modulated electrical signals whose
voltage was determined by the level of reflected light. A synchronizing
signal was included in the transmitted data stream. The scan disk
on the receiving end rotated at the same rate as the transmitter
scanning disk, and an electric lamp's brightness was varied according
to the signal's picture voltage level. As you might guess, the result
was very crude, but it was a first step and did produce a useable
televised image. Shortly thereafter, practical electronic scan
systems were developed that eventually won out as the preferred
television transmit/receive scheme.
* Monel is a metal alloy of nickel and
copper, used for dog tags in WWI , WWII)
Television Forges Ahead
New type of scanning disc embodies features which make it adaptable
to any transmitting system now in use.
By Kenneth A. Hathaway
Photo © The Chicago Daily News
Arthur H. Watson holding the new scanning disc which
he invented and which may hasten the time when television
will be available to everyone
Television development has been given new impetus by the invention
of another type of scanning disc for use in receiving sets, the
first account of which appeared recently in the Chicago Daily News.
The new disc is one of the most flexible that thus far has been
developed, in that it can be adapted to any known system of scanning
that is in use at the present time in experimental work.
It operates on the principle of reflecting light to the screen
instead of projecting it through holes in the disc - a principle
which in itself is not new, although the method by which the effect
is obtained is original with the inventor, Arthur H. Watson, who
heads the Watson Television Laboratories of Chicago.
The design lends itself to manufacture with ease, since there
are no adjustments to be made once the disc has been assembled.
The reflecting surface is made of highly-polished Monel metal. Although
several types of reflectors have been tried nothing better has been
found. The metal was well known during the war when it was used
to make unbreakable mirrors for the troops. As applied to the television
disc, Monel metal has the property of retaining luster, and it can
be bent to fairly sharp angles without injury. Reference to the
illustrations will show that the segments of the disc protrude at
an angle nearing that of ninety degrees.
The complete disc is about seven inches in diameter, a qualification
that permits the use of a comparatively small receiving unit. Aside
from the screws that are used to hold the assembly together, the
disc can be made of four parts: the Monel disc, and a base ring
which carries the bearing, and to which is attached a ring molded
in such a way that each of the segments on the metal disc will be
given a definite angle. In order that the segments will be held
rigidly in position a complementary ring is fastened to the base,
and as it is clamped into position each of the segments takes the
proper angle as determined from the experiments conducted in the
Cross-sectional view of the Watson television receiver,
showing how the mirrors on the rotating scanning disc reflect
light from the crator type neon lamp to the screen, which
may be five by six inches square
Details of the scanning disc, showing how the reflecting
mirrors of highly polished Monel metal are bent to the proper
The mounting of the disc is determined by the method used by
the station transmitting the television impulses. Although attempts
have been made to standardize the television branch of the radio
industry, there seems to be no logical way in which the methods
can be entirely standardized at the present time. Therefore, we
find that different laboratories scan the image horizontally or
vertically. In most of the methods the scanning line describes an
arc, although the position of the arc differs. The ends of the segment
may be at the top or the bottom. Regardless of the position, the
Watson disc can be changed to conform to the transmitting system
by merely varying the angle at which the disc is mounted.
The size of the scanning element employed is seven inches in
diameter overall and one and three-quarters inches in depth. The
picture produced with this element, with the screen at a distance
of fourteen inches, is as large in area as the scanning element
itself, measuring approximately five by six inches. No lenses or
optical reflection or refraction devices are used other than the
segmental elements of the scanning device itself. An entire receiver
may be housed within a cabinet nine inches wide, fifteen inches
high and eighteen inches in length.
Fig. 3 shows the scanning element, consisting of an annulus of
Monel metal, and having inwardly extending radial slots separating
the annulus at its inner periphery into as many segments as there
shall be lines to the picture. This annulus is then cupped to a
curvature on a radius of substantially 366 millimeters, and its
concave face subjected to a single grinding and polishing operation,
thus producing forty-eight perfect and identical optically correct
Next, the segments are struck upwardly by die to an angle of substantially
forty-five degrees from the axis of rotation, being the central
axis of the annulus. The annulus thus prepared is mounted upon a
Bakelite mounting disc, and by the use of a pair of complementary
retaining rings the annulus is drawn tightly in engagement with
the disc. Constantly changing inclination on the opposed surface
of the retaining rings securely locks the segments to their proper
adjusted and operable positions so that the optical axes of the
segments diverge outwardly, each adjoining axis slightly higher
or lower as the case may be. A lamp of the crator type is placed
so that the image of its incandescent gaseous part is projected
upon the screen S, and when the scanning annulus is rotated a succession
of parallel lines traverses the screen.
This system of scanning differs from any of the present known
scanning systems, in that the exact shape and size of the spot traversing
the screen is determined with great accuracy by the use of a mask
associated with the lamp. Formerly a scanning spot of peculiar shape
devised as most efficient could be obtained only through the use
of a special die for stamping the holes in the spiral disc, which
might be expected to result in difficulties arising through the
wearing of the die, not to mention the high cost of the apparatus
necessary for construction.
Fig. 3 - Side view of the scanning disc, showing its
assembly of the rings and Monel metal mirror segments
Salient Features of the Watson System
Pictures five by six inches may be produced directly on the screen
without any intervening means for the purpose of magnification.
It may be instantly adapted to any type of television scanning
systems in use today, an advantage not possessed by any other known
No lenses or optical reflection or refraction devices are used
other than the highly polished reflecting elements of the disc itself.
An entire receiving machine may be housed within a cabinet nine
inches wide, fifteen inches high and eighteen inches in length,
and these proportions may be still further reduced if desired,
The complete televisor comprises but four parts, the lamp, the
drum, a motor for driving the drum and a screen.
The problem of synchronization is taken care of in metropolitan
areas by using a synchronous motor, and swinging the entire motor
and drum along the axis of the drum to adjust for phase position
between the poles of the motor, the major adjustment being carried
out by momentarily breaking the motor circuit, permitting lag to
take place. For intra-area reception a variable speed motor may
During the months in which Mr. Watson has been developing his
disc he has tried other types of reflecting material. Nickel-plating
was one of the methods tried, but nickel-plating fails to hold its
luster when exposed to the atmosphere and requires frequent polishing.
The extensive use of chromium plating appeared to present another
possibility, but it was discarded when it was found that imperfections
in the metal would show through the plating in the form of pockmarks.
The marks would not be apparent to the eye, but when the light was
reflected upon the screen they were very much in evidence. Using
chromium plating over a layer of nickel failed to serve the purpose,
for the imperfections in the surface of the metal were made evident
through the double plating.
Accordingly, the inventor reverted to Monel metal, which reflects
with nearly the same efficiency as silvered glass. At the same time
the metal is lighter and more serviceable.
Posted March 27, 2014