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A magnetic field is manifested by a force affecting certain materials in its environment,
acting on moving electrical charges. The magnitude of this force decreases as a function
of the distance between the magnetic field emitter and its action point. Magnetization
is a vector quantity which characterizes on a macroscopic scale the magnetic behaviour
of a material sample. It is measured in amperes per metre (A/m) or in Tesla (T).
Ferromagnetism designates the ability of certain material samples to become magnetised
under the effect of an external magnetic field, and to retain part of this magnetisation.
On a microscopic scale, a ferromagnetic material can be described as a set of independent
magnetic domains (Weiss domains).
When the magnetic moments of the domains are not aligned, the total magnetostatics
energy of the material is minimised: the material is not magnetised.
(2)
For demagnetization purposes, a damped magnetic excitation field is applied so
that certain domains are reversed more than others, depending on the magnetic
excitation force passing through them. This leads to a state of magnetic disorder
equivalent to the state of a non-magnetized material.
(4)
There are relatively intense magnetic fields all around us in daily life. It goes without saying that mechanical watches are
influenced by them, and thereby undergo potentially significant operating variations, i.e. which affect the rate precision.
It should be noted that the Earth's magnetic field varies between 25 and 65 μT, depending on geographic position.
The most significant items in everyday life which can affect the rate of a mechanical watch by means of a magnetism effect
are computers, mobile phones, TVs, refrigerators, etc. Magnetic flux measurements (taken at 10 cm from the source) have
revealed values of 60 μT for an electric toothbrush, 75 μT for a mobile phone and 110 μT for a computer. Consider too that
watchmaking tools often have a magnetic charge capable of disrupting the rate of a watch !
The distance at which a watch is placed from the magnetising object is an important factor. Although watch manufacturers
regularly equip their cases with protective materials or opt for non-magnetic components, nevertheless rate problems due
to magnetism remain very common.
1.
THE MAGNETIC FIELD
(3 and 4)
3
(1)
(1)
(2)
(3)
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