[time-nuts] Antique precision timing device without electronics

Morris Odell vilgotch at bigpond.net.au
Thu Mar 16 05:22:42 EDT 2017

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



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