That strikes a chord

I have been asked to make some photographs of a handful of "mystery objects". This is one of them and is my favourite. I recognise it - my dad had one of these, in far nicer condition - polished to an ivory finish by years of use. 

Trained as a draughtsman or technical drawer in the steel industry (I'm not sure what exactly) in his early years, his tools and paraphernalia supported his hobbies later in life. My dad was a great maker of things and I believe that is where my creative gene arose. When I was not "helping" him in his darkroom, I was "helping" him with his tools for woodworking or whatever or watching, fascinated (no little fingers allowed) as he cut tiny pieces of veneer for a piece of marquetry.

I grew up with a great feeling for hand tools and scientific instruments. In time I "inherited" his mathematical instruments as I struggled my way through O-level Maths. I never lost my love for such things, or my fascination with a well-used beautiful hand tool, handles worn to a satin finish by the palms of grandfather, father and son in their turn.  If I see a plane, a drill or a screwdriver in an antique shop I have to pick it up and handle it, feel its heft. Should it be made with mahogany handles and brass fittings I feel a terrible urge to have away with it and make it mine. 

I mourn the replacement of proper poky DIY shops with the modern sterile equivalent of B&Q and still recall Saturday morning shopping trips to "Stan's" our local DIY shop and its wonderful smell of sawdust...

But I digress. This measuring or calculating tool is in need of identification. I have had a little Google and believe that it may be some kind of logarithmic calculator perhaps similar to a slide rule and related to Gunter's Scale, I think. It clearly has some kind of function in measuring or calculating chords (is marked  with CHO) and has protractor-type markings. Whatever it is, I have enjoyed the trip down memory lane and imagine my delight to find this

I was looking back for a right angle tool that I had photographed last year and was certain that I had blipped. A little over 12 months ago my Blips were all about the DIY... but no, I can't find it - so I'll add it as a bonus extra. Just  visual evidence of my obsession with tools and scales and measurement.