by Walt Odets

Light-wound clockNearly
a half century ago–in 1950–Patek Philippe began marketing an
extraordinary invention, the “light-wound” table clock.
The clock required no regular winding and, after full charge,
was capable of running in complete darkness for a year. Furthermore,
Patek claimed a very impressive accuracy of within one second
a day.

To place the light-wound clock in historical
perspective, 1950 was also the year that Patek introduced the
Gyromax balance wheel, which at the time seemed an important
development for the future of the wristwatch. It would be only
two years later that Patek would introduce a “fully electronic
clock, i.e. without moving parts,” and still another year–1953–before
Patek introduced its first automatic wristwatch. Just a year
after the automatic Patek introduced the first “nuclear-powered”
timepiece, “deriving its energy from a radio-active isotope.”
And finally, in 1958, Patek produced its first quartz-controlled
clock. This is an interesting history for a company that is,
today, so strongly associated with conservative and traditional
mechanical wristwatchs.

Circuit diagramA

By contemporary standards, the Patek light-wound
clock is a technological anomaly and peculiarly primitive. It
combines what were, in 1950, state-of-the-art electronics (including
very expensive photoelectric cells) with a traditional Patek
mechanical hand-wound movement, a modified caliber 17”’-150.
As shown in the schematic at left, a photoelectric cell
(lying under the top dome of the clock case) was used to charge
a capacitor (“accumulator”), that, through a mechanical
switch, powered a motor that–wound up the watch! As illustrated
below right, the components of the clock were assembled
on a vertical plate behind the dial and a horizontal plate that
carried the movement, the switching mechanism, and a “demultiplication”
gear train that reduced motor speed for winding. The movement
is indicated at 1, the winding motor at 2, the
demultiplication gear train bridge at 3, the
Back viewmotor switch plate at 4, the storage capacitor
at 5, and the back of the dial at 6. The hand-setting
knob is shown at 7.


The caliber 17”’-150 is a 17 ligne (38.25
mm), 2.90 mm tall, 18,000 beat per hour hand-wound movement.
It runs in 18 jewels, without shock protection. While the earlier
version of the clock used a micrometric swan’s neck fine regulator,
later versions of the clock were equipped with a Gyromax balance
and free-sprung overcoil hairspring (without regulator). In the
light clock, Patek specifies the normal amplitude at “slightly
under 270 degrees measured with the minute hand set at 6 or 12
o’clock.” This is necessary because the amplitude increased
between 2 and 5 o’clock due to the “falling” force
of the relatively heavy, vertical-mounted minute hand. The frequent
motor winding action also caused an increase in amplitude. Because
of both influences, an intial amplitude of 270 to 310 degrees–a
normal range for most healthy watch movements dial down–would
have produced excessive amplitude and knocking of the escapement.
As illustrated at left (with the winding train bridge
and mainspring barrel gear removed), the movement is a conventional
Patek full-bridge movement.
MovementThe earlier traditional micrometric regulator
is illustrated here (10). In the later Gyromax version,
as with any Patek watch so equipped, a quarter-turn of a pair
of balance weights provided about a seven second correction of

The conventional nature of the movement
ends at the center wheel. As illustrated at right, power
transmission from the (horizontal)
center wheel to the (vertical)
motion work of the hands is accomplished with a bevel gear
(1) and cannon-pinion spindle Bevel gear and setting pinion(2). Because
the bevel gear requires considerable backlash, the backlash that
would otherwise be visible in the hands is eliminated with a
Teflon “hand brake” (3), a sophisticated
material for 1950. The Teflon brake, Patek instructs, “acts
without oil.” The hand-setting knob (4) is carried
on the rearwards extension of the cannon-pinion spindle.

Reduction gearingThe
winding train and mainspring barrel are, of course, also unconventional.
As illustrated at left, the motor shaft drive gear (5)
drives a series of reduction gears (4, 3, and 2),
which wind the mainspring barrel (6). This gear train
is not unlike an enlarged version of the kind of reduction train
that exists between the rotor and barrel in an automatic watch.
In both cases, the train reduces the high-speed, low-torque output
of the rotor (or motor) to the slow-speed,
high-torque input needed to wind the
mainspring inside its barrel.

Mainspring barrelThe spring barrel, as illustrated in detail
at right, provided the connection between the mechanical
movement and the electronics. Instead of a ratchet wheel, the
otherwise conventional mainspring barrel (1) carries a
driver (3) and release nut (2). The
release nut carries a large wheel driven by the pinion gear of
2, as illustrated at left. (The large wheel on
the barrel is removed in this illustration.) The
Barrel and switchangled ring at the base of the
driver and release nut (red arrow) carries the forked
extension of one of the switch arms. As the barrel drives the
movement, it rotates clockwise (as seen from the top plate, as
illustrated), gradually raising the forked arm (onto the higher
section of the angled ring) and closing the switch contact. Approximately
one-half unwinding rotation of the barrel initiates rewinding.
Consequently, one half turn of rewinding moves the fork back
to the lower section of the ring, and winding is stopped.

As a full wind represents approximately
five turns of the barrel, the mainspring tension is thus kept
relatively constant. Because such tight regulation of mainspring
tension diminishes any effects of deficient isochronism, this
close regulation of mainspring is certainly an important contribution
to the running regularity of the clock. A permanent horizontal
(dial-down) position and stationary use are others. During vibration
and timing, the movement could be adjusted for optimum running
in a single position. Patek advised that daily rate could be
observed by noting the position of the minute hand, but always
in relation to the same point on the dial (to avoid errors introduced
by irregular graduation on the dial). As a preferred alternative
method, a mark was engraved on one arm of the fourth wheel (the
“seconds wheel,” revolving once per minute) to index
against an adjacent fixed point in the movement. This method,
Patek advised would allowing adjustment of rate “to the
nearest second.”

The Patek light-wound clock is a very impressive
piece of work for 1950. And, in some odd way, it is even impressive