[time-nuts] Antique precision timing device without electronics

Bill Hawkins bill.iaxs at pobox.com
Thu Mar 16 14:04:40 EDT 2017


Hi Morris,

If there's no active devices (and you'd be sure to see them, not solid
state) where does the power to operate the motor come from? Is it the
same contacts that drive the fork?

It's amazing that there is high Q when contacts must be operated by the
fork.

Did it come with instructions for setting the weights at the end of the
fork tines?

Best regards,
Bill Hawkins 

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Morris
Odell
Sent: Thursday, March 16, 2017 4:23 AM
To: time-nuts at febo.com
Subject: [time-nuts] Antique precision timing device without electronics

Hi all,

I was recently asked to resurrect this interesting device by a colleague
who collects antique scientific instruments. It's a "Chronoscope" made
by the H. Tinsley company in London in the early 20th century and used
to measure time intervals with the precision of those days. It's large
and heavy in a polished wooden case with a top deck that hinges up to
reveal the innards. 

The timing reference is a large tuning fork about 30 cm (1 foot) long
and running at 25 cps. It's normally in a glass fronted housing (removed
for the video) that includes a pair of hinged mechanical arms for
starting it. It's maintained in oscillation by an electromagnet and
contact arrangement powered from a 12V DC supply. The fork amplitude is
controlled by a rheostat - too much and the tines impact on the magnet.
The video frame rate makes the fork look slower than it actually is. I
was able to extract a signal and measure the frequency with a modern GPS
disciplined counter - it's 0.007% off its specified 25 Hz! The frequency
is too low for my HP 5372A so I was not able to easily get an idea of
stability or do an ADEV measurement. The fork has quite a high Q and
takes over a minute to stop oscillating after the power is turned off.
There's a built in higher voltage AC power supply, probably a mains
transformer, potted in beeswax in a polished wooden box inside that is
intended to
  energise a large neon strobe lamp used to adjust the fork.
Unfortunately the lamp was not with the unit and is no doubt
irreplaceable. 

The 25 Hz signal is filtered by an LC network  and used to run a
synchronous motor in the Chronoscope unit. Synchronous motors not being
self-starting, you need to tweak a knob to get it going - there's a joke
in there but I can't for the life of me think what it could be ?? The
"Contact" switch and associated socket on the back controls an
electromagnetic clutch that connects the clockwork counter mechanism to
the motor and the contact "on" time is indicated on the dials with 10 mS
resolution. 

There's not a single active device in there and after a clean and lube
it runs very nicely from a modern 12V DC plugpack. My friend is very
pleased with it and it will take pride of place in his collection. 

I'd be interested to know if any time nuts have knowledge or experience
of this lovely instrument.

A video of it is at  https://youtu.be/i5S8WS9iN_E

Enjoy!

Morris

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