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

Chuck Harris cfharris at erols.com
Fri Apr 18 02:04:14 EDT 2014

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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