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

Tom Van Baak tvb at LeapSecond.com
Wed Apr 16 13:43:55 EDT 2014

> 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:


> 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:
output to scope:

> 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.

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