Re-magnetising a Bulle magnet

What has to be achieved

A Bulle bar magnet is not an ordinary two-pole magnet. It has three poles: the same pole at both ends and the opposite pole in the middle, normally south–north–south.^[3] Re-magnetising has to recreate that pattern. A magnet given a simple north and south by someone who did not know this will not drive the clock properly. This happened to Bulle Nº 11536, whose notes record that “someone has re-magnetised it but did not understand the working of a Bulle.”

In principle, the pattern is made by magnetising each half of the bar separately, with the field running from the end towards the centre, so that the two halves meet as a like pole in the middle. The field applied has to be strong enough to exceed the coercivity of the magnet steel, the point at which its magnetism can be fully reset.^[4] Where two poles sit side by side on one piece of steel, the change from one to the other is never perfectly sharp, which is one reason the centre of the bar needs care.^[4]

Ways it is done

Magnet makers use three main kinds of equipment: large direct-current electromagnets, “half-cycle” magnetisers that draw thousands of amps from the mains for a fraction of a cycle, and capacitor-discharge (impulse) magnetisers that release stored energy through a coil in a single controlled pulse.^[4] Modern impulse magnetisers use electronic switching and are the safest and most controllable way to deliver the necessary pulse.^[4]

In amateur workshops the same effect is often improvised with heavy coils and a car battery or a home-built capacitor unit.^[2] These can work, but they concentrate a great deal of energy in exposed wiring and terminals. Beyond the electrical risk, wet lead-acid batteries contain corrosive acid and release hydrogen when charging, and sparks near them can cause them to explode.^[1] For that reason this page does not give instructions for improvised methods.

The Horologix restoration notes describe magnets being “zapped” back to strength (Bulle Nº 5962) but do not document the equipment, so no particular device is recommended here.

Checking the result

Whatever method is used, the magnet should be checked in two ways before it goes back in the clock:

• Polarity. Pass a compass along the bar. Both ends should attract the same end of the needle, and the centre the opposite end. On Bulle Nº 183374 the ends had been scratched “S” and “N”, but a compass showed both were south, as they should be.
• Strength. Measure the magnet on the magnetometer. A standard tall-movement magnet should reach 180 mm or better; weak magnets typically read 140–160 mm before treatment.

North–south–north works just as well as south–north–south, provided the battery connections are reversed to suit (Bulle Nº 174475). The later two-pole cobalt magnets, as on Nº 342201, cannot always be brought back to their original strength.

After re-magnetising

Keep the magnet away from steel tools and other magnets, which can weaken it again. Once it tests correctly it is cleaned and finished with fire-grate black rather than paint, because it is handled a great deal during setting up and paint soon scratches.

← Back to documents