[time-nuts] Measuring the accurcy of a wrist watch

Tim Shoppa tshoppa at gmail.com
Fri Apr 18 13:52:25 EDT 2014


At least for low-end quartz+mechanical watch movements, magnetic fields can
cause the watch to stop mechanically ticking or even produce false ticks to
cause dial to spin at a furious rate.

e.g. By holding my watches at a certain angle in the field of an AC tape
degausser, I can make them run many times faster than normal, and I had one
mechanical watch where if I held it at the funny angle it would actually
run backwards! (Must've been something other than a one-way escapement
inside I guess.)

This is not the quartz crystal and electronic divider being affected by a
magnetic field, this is the stepper motor and mechanics turning in response
to magnetic fields.

It does not take a magnetized guitar pickup to catch the magnetic field
coming from the stepper coil in the watch. Any nearby unshielded coil or
AF-range inductor will pick up easy.

Tim N3QE


On Fri, Apr 18, 2014 at 2:04 AM, Chuck Harris <cfharris at erols.com> wrote:

> Early watches were more susceptible to magnetic influence than
> were later... this is primarily because the early watches used
> high carbon steel hairsprings for the balance wheel, and when
> they got magnetized, the spring coils would stick together...
>
> Later watches used elinvar for the hairspring coils because its
> spring constants were less affected by temperature variations...
> a nice side benefit is it is not easily magnetized.
>
> However, when an elinvar hairspring gets magnetized, it is very
> difficult to demagnetize it using conventional means.
>
> Demagnetizers work by rapidly alternating the polarity of the
> magnetic field, and slowly decreasing the strength of the field.
> This causes the magnetic poles of the ferrous atoms to get randomly
> aligned, which is the demagnetized state... But if the item that
> is magnetized is so light weight and flexible that it can move
> with the field, it won't get demagnetized... which is what happens
> with the hairspring.  The only way I know to demagnetize a hairspring
> of this sort is to immobilize the spring with wax, and then run it
> through the demagnetizer... then melt the wax, and clean the spring
> with naptha.
>
> Fun times!
>
> -Chuck Harris
>
>
> DaveH wrote:
>
>> I remember growing up (50 years ago) that the good watches were marked as
>> being non-magnetic.  I would guess that this is standard now.
>>
>> My concern is that the moving balance wheel could have an eddy current
>> induced into it and the resulting magnetic field might cause it to slow
>> down.
>>
>> http://en.wikipedia.org/wiki/Balance_wheel
>>
>> http://en.wikipedia.org/wiki/Eddy_current
>>
>> The act of measurement should not cause a change in what you are
>> measuring.
>>
>> Dave
>>
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