The Clock and its Operation
The Clock and its Operation
The clock is based on the reaction between a permanent bar magnet and an electric coil, hence it is classified as an electromagnetic clock.
In the Bulle clock the coil forms part of the swinging pendulum.
When the pendulum is in motion, it makes and breaks electrical contact at the pin and the fork.
Current flows for the split second of contact, activates the coil and produces a magnetic field around the coil.
This magnetic field opposes the field in the bar magnet, which pushes (impulses) the coil, hence pendulum swings away from the central magnetic pole.
The Bulle magnet is unique and hard to believe it can exist. Made of cobalt steel, in either an arc or U-shape, it has three poles !
This unique feature produces a very high magnetic field at the centre of the bar and at this point the lines of force are perpendicular to the axis of the magnet.
When the coil is activated at the centre point of its swing, ie. the centre of the bar magnet, the coil’s field cuts the magnetic field to create the opposing reaction and drive the pendulum away.
Please note that the diagram to the left is actually incorrect. The lines of force from the coil do not radiate outwards as shown (“Claimed”). The lines of force align themselves along the coil, and work at right angles to the magnet’s field (“Actual”).
This is shown in the additional sketch.
It is also worth noting that the unique magnet design is also its downfall. The design is such that eventual demagnetisation is a certainty, because the magnetic domains at the centre of the magnet are in constant opposition.
Bulle Clock Technical Construction Details
While it is not immediately apparent, the Bulle clock is polarity sensitive. If the battery is connected the wrong way around, the clock will not run. This is determined by the direction in which the coil is wound, and the magnet. The very first test of a newly acquired Bulle will reveal if the battery is connected properly. If the clock won’t run, try reversing the battery polarity first. Further work is needed only if this fails to make the clock work.
The silver pin and the fork contact must be absolutely clean. Even a small amount of tarnish will stop the clock, because of the low voltage and minimal contact pressure.The silver pin and the fork contact must be absolutely clean. Even a small amount of tarnish will stop the clock, because of the low voltage and minimal contact pressure.
Here you can see the sequence of events as the pendulum swings. The pin makes contact only with the uppermost corner of the fork contact, and the contact period is very short. I have measured it at ~190ms, although the actual time will vary somewhat depending on the way the clock has been set up. A small variation is to be expected, and it will also vary depending on the pendulum period (714ms for the one I measured, representing an 84 beat/minute pendulum). It is probable that if the contact duration is around 25% of the pendulum period the contacts are reasonably well adjustmed.
Make sure that the silver contact spring, its rolling loop and the loop pin are also scrupulously clean. These points must never be oiled because the oil film can create an insulating barrier which will prevent current flow.
To compensate for variation in battery voltage, the Bulle clock is fitted with an isochronism corrector which, by its arrangement, causes a retarding force on the pendulum with each oscillation. This is zero to start with but increases with the amplitude and, adding to the force of gravity, enables the clock to maintain a constant rate in spite of amplitude variation.