By Bob Holmström

                     Marcel Bétrisey is a clockmaker in Sion, Switzerland. Marcel says that he may be a
                     strange clockmaker because he prefers to use controlled “hazardous” effects instead
                     of precise effects to drive his clocks. He uses puffs of air, moving balls, etc., to create
                     his works of art that also tell time. His web site at www.betrisey.ch shows many
                     examples of his work.

                     The Chronolithe, with its radiometric pendulum is different from his other clocks in that
                     the driving force is light. This force is provided by two pairs of 5 watt halogen lamps
                     placed on either side of the pendulum that alternatively illuminate “flags” on the
                     pendulum rod to push the pendulum. This “motor” is simple, and it appears that
                     nobody has employed it before for driving a pendulum. Marcel states that the lamps
                     act not only to push the pendulum, but also act as a brake thus controlling the
                     pendulum amplitude to a very precise range. The clock is composed of a glass tube,
                     the pendulum assembly, two photoelectric cells, two relays and the stepper motor
                     from a quartz crystal controlled clock.

                     The pendulum has an Invar rod and the bob is a stainless steel sphere filled with lead.
                     The total weight of the pendulum is 2.3 Kg. The pendulum is supported by a
                     conventional suspension spring in the interior of a tube that is at a vacuum with a
                     pressure of approximately 0.01 bar. The frequency of oscillation is 0.5 Hz with an
                     amplitude of 5 millimeters. The flags are grey mica sheets. The black side is coated
                     with lamp black from candle smoke. Marcel used 4 Crookes radiometers with the
                     vanes rotated because they were a convenient source and mounting for the mica
                     material. The forked extension located between the flags is used to guide the
                     pendulum assembly into the tube without damage.

                     The pendulum is started by use of a magnet that is moved close to glass. The
                     adjustment of the period is done from the outside by turning the 4 planetary spheres
                     as a group around the sphere of the pendulum bob using the same magnet. This
                     avoids having to let in air, dissemble the clock and re-evacuating the air for each
                     adjustment.

                     The illustration on page 13 shows the performance measured using a MicroSet for a
                     period of 2 ½ days. Measurements were made every 10 seconds. Marcel thinks that
                     the short term instability shown is due to inaccuracies in the infared sensor/receiver
                     and cats’-eye reflector used to control the lamps.

                     The basic principle of Chronolithe’s operation was demonstrated by Sir William
                     Crookes 120 years ago. Crookes found that the force was proportional to the
                     intensity of the illumination. The force also increases with gas pressure to a maximum
                     and then falls rapidly. The pressure at which the peak occurs decreases as the flag
                     dimensions increase. The mechanism by which it operates is very complex and is
                     frequently stated incorrectly. The accepted explanation involves the concept of
                     “thermal creep” proposed by James Maxwell and Osborne Reynolds.

                     Kennard in Kinetic Theory of Gases says: “.. A tempting hypothesis at first sight is
                     that the radiometric force is due to the reaction from gaseous molecules rebounding
                     with higher velocities from a hot surface than from a cold one; but this is quickly seen
                     to be untenable when we reflect that such molecules, upon reentering the gas, must
                     drive it back and thereby thin it out until the pressure is reestablished, whereupon the
                     force on the hot surface will become the same as on the cold one and no radiometric
                     action can occur. The cause must, therefore, be sought in some secondary action.
                     The effect has very commonly been regarded as occurring at the edge of the
                     radiometer disk, where condition in the gas must be far from uniform; experiments
                     designed to show that it is simply proportional to the length of the perimeter failed,
                     however, to yield this result. Recent theoretical and experimental studies have now
                     made it pretty clear that most, if not all, radiometric phenomena are due, in one way
                     or another, as Maxwell suggested in 1879, to the thermal creep of the gas over an
                     unequally heated solid. … It can be seen easily that this creep must give rise to
                     forces on the surface whenever the resulting flow of gas is hindered …. by viscosity,
                     and consequently the gas accumulates somewhat over the blackened surfaces and
                     exerts a slightly increased pressure on these and so pushes them back, ….”

                     The action of the Chronolithe pendulum is also effected by the alternative heating and
                     cooling of the flags as evidenced by the following statement from a reference on the
                     Web: “Why a radiometer runs backwards after the light is turned off - Heat escapes
                     quickly from the black sides of the vanes. Thus, the black molecules cool off first.
                     Meanwhile, the white molecules take longer to lose heat and cool down. The result is
                     that gasses from the white vane push off with more force (Newton’s third law) and the
                     vanes spin in the opposite direction.” Chronolithe is now on loan to the International
                     Museum of Horology in La Chaux de Fonds, Switzerland. Chronolithe won the first
                     prize in July 2002 International Kinetic Art Competition.

                     Marcel ends the description of Chronolithe on his web site with the statement: “I’ve
                     learned a lot with this clock. The most difficult thing was always to keep the look I
                     wanted. It would have been much easier to build something bigger, with a bigger
                     glass pipe, without the stone base, without all those vacuum tight problems... The next
                     clock will be different: I want to make the most precise clock I can with the same
                     radiometric principle, and this time I won’t care about aesthethics. At least it’s what
                     I’m thinking now…

                     Marcel has now constructed his second radiometric clock. It is called Conti and
                     Marcel says that it should surpass Chronolithe since it has inherited all the knowledge
                     that Chronolithe slowly distilled into him during its construction.

                     Conti is designed to be superior in sealing and precision. It is superior in sealing
                     because it has only one joint at the bottom of the tall glass dome and no internal
                     clamping rods. It is superior in precision because it is controlled by laser and not by
                     infra-red sensors. It is also much more practical to work on and to adjust.

                     The driving force of the pendulum is the same as that of Chronolithe. The hands for
                     the hours, minutes and seconds are moved by a stepping motor connected to two
                     small solar panels that are illuminated at the same time the pendulum is impulsed.

                     The glass dome enclosing Conti is 1300 mm high, 200 mm wide and has a thickness
                     of 20 mm. The pendulum rod is Invar. The pendulum is a sphere of stainless steel
                     filled with molten lead and weighs approximately 4 kg. The base is a massive piece
                     of bronze.

                     Two laser beams “look” at the pendulum rod. When it passes between them, the
                     lamps in one side are turned on (depending on the sense of the swing). The lower
                     lamp moves the pendulum, the higher lamp powers a solar panel which moves the
                     magnet of the stepper motor for the seconds hand. The main difference from
                     Chronolithe is that the lamps are only on during the period when the pendulum rod
                     blocks the laser beams.

                     Without the benefit of horological or scientific training, Marcel Bétrisey has taken the
                     concept of the Crookes Radiometer and transformed it into a precision timekeeping
                     device with great potential. If his work had been done when the pendulum was the
                     standard of timekeeping perhaps the history of precision horology would be very
                     different. Marcel has now turned his attention toward Foucault pendulum clocks – see
                     his web site www.foucault.ch for details.

                     Reference list:

                     Orginal papers by Maxwell and Reynolds: “On stresses in rarefied gases arising from
                     inequalities of temperature” James Clerk Maxwell, Royal Society Phil. Trans., (1879).
                     “On certain dimensional properties of matter in the gaseous state” Osborne
                     Reynolds, Royal Society Phil. Trans., Pt. 2, (1879).

                     History of the Crookes radiometer: “The Kind of Motion that we Call Heat” S.G. Brush
                     North-Holland, 1976.

                     “William Crookes and the Radiometer” A.E. Woodruff, Isis, Vol. 57, No. 188, 1966,
                     pages 188 - 198.

                     Text books with coverage of the theory involved: Kinetic Theory of Gasses, Earle
                     Kennard, McGrawHill, (1938).

                     Handbook of Vacuum Physics, Volume 1 Gases and Vacua, Part 5 – Kinetic Theory
                     of Gases and Gaseous Flow,
                     J.D. Swift, Pergamon Press, 1966.

                     The Kinetic Theory of Gases, Leonard B. Loeb, Dover Publications, reprint of 3rd
                     edition, 1927.

                     Recent useful references: “Concerning the Action of the Crookes Radiometer”
                     Gorden F. Hull, American J. Phys., 16, 185-186 (1948).

                     “The Radiometer and How it Does Not Work” Arther E. Woodruff, The Physics
                     Teacher 6, 358-363 (1968).

                     “Crookes’ Radiometer and Otheoscope” Norman Heckenberg, Bulletin of the
                     Scientific Instrument Society, 50, 40-42 (1996).

                     “How does a light-mill work?” Philip Gibbs 1997
                     http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html.