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

Chris Albertson albertson.chris at gmail.com
Wed Apr 16 18:55:37 UTC 2014

I just did an experiment.  Place a simple quartz movement wrist watch on
top of a Fender Stratocaster guitar.  I get a very strong and easy to
detect signal.  A loud and sharpt "ping" once per second.  More then 1 volt
peak to peak.   I can cancel almost all the background hum and hiss in the
normal way by using the selector switch on the guitar.

The guitar has a pickup coil with many thousands of turns of #40 wire.
 With the selector with at #2 position there is a second coil some inches
away that is wound in the opposite direction and the two are added
canceling any field that is filing the room.

I tried the same with a wall clock and all I had to do was hold the clock
an inch away.  The wrist watch was placed on top of the strings a few mm
above the bridge PU.

These is likely about 3 oz of #40 magnet wire on a guitar PU.  If I were
building a sensor I'd do it just like the guitar.  one coil to pick up the
signal and another identical coil some inches away to to pick up ambient
"noise" and then wire the two in parallel but in anti-phase.

If yu happen to have a guitar around, you have a watch sensor.

On Wed, Apr 16, 2014 at 10:43 AM, Tom Van Baak <tvb at leapsecond.com> wrote:

> > Tom,
> >
> > can you explain what exactly you understand by "a large coil of wire"?
> Sorry, by large I meant a large number of turns; the coil itself is quite
> small. Rather the winding one myself I just used the pickup coil from an
> old cheap plastic self-impulsed pendulum clock. The wire is extremely fine
> and there must be thousands of turns since the spool diameter is only
> 15-20mm and the net resistance is 3.5k. Here are some iPhone photos I just
> took:
> http://leapsecond.com/pages/Junghans/coil.htm
> > Did you make the easurements on the Junghans with a DIY sensor or with
> one
> > of the commercially available?
> Both. The commercial ones sold by Bryan Mumford are excellent; his
> instrument includes signal conditioning, adjustable high gain, and other
> useful features. It's meant for watchmaker types with no electronics
> background. It works perfectly out of the box.
> The Junghans wristwatch is extremely well engineered for long-life and the
> leaked magnetic signal is the weakest of any watch I've measured. Still, it
> can be measured. The placement of the pickup coil on the watch face needs
> to be optimized for best "reception", or any reception at all for that
> matter.
> By contrast, a typical AAA-battery desk/wall quartz clock movement
> generates a huge magnetic signal. It is so clean that you can clearly see
> both the start (+) of the impulse and the end (-) of the impulse about 30
> ms later. In fact I suspect it's actually 31.25 ms, or 1/32 s, since that's
> 1024 cycles of a 32.768 kHz oscillator. See:
> sensor placement:
> http://leapsecond.com/pages/Junghans/quartz-clock.jpg
> output to scope:
> http://leapsecond.com/pages/Junghans/coil-aa.gif
> > I have made some basic tests with a coil coming from a loudspeaker's
> cross
> > over network. It has a few hundred windings, R=1.3 Ohms, 2.3 mH, but the
> > only thing i receive with this coil is a strong 10 Mhz signal...perhaps
> no
> > real surprise in a time nuts laboratory.
> I suspect your 1.3 ohms means the number of turns is far too low.
> I don't see any RF here, nor even very much 50/60 Hz.
> /tvb
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Chris Albertson
Redondo Beach, California

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