Using Vernier calipers

You might recognise the contraption above as my self-designed Limpet Height Measurers from your first year trip to Dartmoor (patent still pending). But I also often thrust them upon people who are collecting snails, spiders, beetles and so on, because trying to measure these creatures can be a lot easier with Vernier callipers than grappling with a ruler. Sometimes that's because rulers don't easily fit in petri dishes, or because 3D objects don't easily sit against a ruler, or just because it saves you holding something small and fiddly while trying to squint at a tiny scale at the same time.

The distances between the red arrows above are all the same, though mostly you'll probably be using the bottom left gap (for snail width or spider body length for example). Most of the time you probably won't need the precision of the Vernier scale at all - millimeters being good enough for most things of this nature, but here's a quick reminder of the whole process.

Being a proper scientist you'll be working in mm or cm not inches, so use the scale on the lower side of the calipers. You want to read your distance off against the line above the O (some people get confused and want to read where the gap of the jaws ends).

So this is somewhere between 1.7 and 1.8cm (17 and 18mm), would you agree? That might be good enough for your purposes (if the question in your key is 'Over 15mm or Under 15mm, for example).

But if you did want to know how many bit-ths of a millimeter it was, you must now forget the main scale and scrutinise the 0, 1, 2, 3 etc Vernier scale.

You ask yourself, which of the lines in that scale matches up in a straight line with one of the lines above it? I would say all the ones at either end are quite askew, but the best choice is at 5 and a half - yes?

So that means after our aforemeasured 17mm, there is 0.55 more of a millimeter, making 17.55mm in total.

That would be a bit over the top for most things, but with some groups you might get a key that requires you to know the measurement to the nearest 0.5 of a millimeter (for example, the descriptions of sedges that I was looking at yesterday talk about the leaves being 1.5-2 mm wide, or the fruits being 3.5 to 5mm long).

 There are of course digital Vernier calipers these days, but I ask you, Where is the Fun in That? You wouldn't know if they were lying to you or not. Likewise their batteries are apt to die at the most inopportune moment (and they are always some obscure size of watch battery that you haven't got). It probably seems strange for a technician to be a Luddite (maybe I need to speak to a therapist). But I maintain this is borne of bitter experience: simplicity is often better. Besides, you will learn a skill which you can take satisfaction in. Here endeth the sermon. If you'd like to borrow some calipers do come and ask. I might even lend you the digital ones if you want.

CC image by Lookang

(If you want to know how a Vernier scale works, and who wouldn't, you could do worse than reading this anonymous blogger's website.  Mr Vernier himself was a French mathematician that lived c.1600.)