How big is a spanner? They all have some markings on them, but it's not always the gap between the jaws. For AF spanners the number written on the spanner is the gap between the jaws in inches (ignoring a little clearance so that it will actually fit over the bolt). For Metric spanners it's the same, but in millimetres. For Whitworth spanners the number is the size in inches of a hole, through which the shank of a well designed bolt would fit, and the head of that bolt would fit between the jaws. For BA spanners it's an arbitrary, usually even number that gets bigger as the spanners get smaller.
That's all well and good, but what does a measurement of length really mean? You've got a ruler, but you just assume that it's got the same measurements on it as all other rulers, how can that possibly be organised? Where did the first ruler come from? The factory that made your ruler had their tools calibrated in a laboratory. That laboratory had their tools calibrated by the National Physical Laboratory (or your country's equivalent) and the NPL compares their reference length against two scratches on a bit of copper stored very carefully in Paris. We've all just sort of quietly agreed that those two scratches are a good distance apart. When it comes right down to it, describing the size of something is just comparing its size to something else.
I had to buy a socket in the wildly inconvenient size of 1 and 5/16 of an inch (AF), while it does have this number on the socket itself, the packaging has a far less useful (although somehow more honest) description of the size. It says "Classic size for Hub Nuts on Mini, MGB and Triumph TR4". It's not a measuring convention that I want to start using in my tool box.
Richard "0.01959 Smoot" B
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