The humble brass sliding gauge is a powerful tool, and no self-respecting Shoebox Studio should consider being without one. I have two. When one goes with me to teach classes, the other stays at home snug on my bench, just in case the first one succumbs to a fit of wanderlust and goes astray….
I used this tool for an embarrassing number of years before I learned to read it properly. Particularly embarrassing because it’s so ridiculously easy.
The muscle resides in the elegance of the Vernier scale.
Notice how the etched lines on the lower edge of the scale do not all match up with the lines above them? When the gauge is fully closed, only the first line (zero line) and the last line have matches. The last Vernier line matches up with the 9mm mark on the metric scale above it, making the Vernier scale slightly shorter. This is so incredibly BRILLIANT, because it allows you to measure precisely and accurately down to 10ths of a millimeter.
I close the gauge carefully on the girdle (widest part) of a small faceted amethyst. Immediately I can see that the stone is larger than 5mm in diameter, because the zero line on the Vernier scale (the one closest to the stone) is pointing to the right of the 5mm mark on the metric scale.
And now, for the Vernier MAGIC! Look at the other marks on the Vernier scale, and choose the one short line that matches up exactly with one of the lines on the metric scale above it. In this case, it’s the 4th short line. This means that the stone is 4 tenths of a millimeter larger than 5mm, so it’s 5.4mm in diameter.
This brass sliding gauge has a metric bias, so the Vernier scale is only calibrated to work with the metric scale on the bottom. On the imperial scale in inches along the top of the gauge, you can tell that the stone is a bit bigger than 3/16″ in diameter, but after that you’re just guessing.
In order for your measurement to be accurate, you need to be sure that you are holding the stone properly in the jaws of the gauge. The easiest way to do this is also the easiest way to pick up a stone with the gauge.
Set the stone near the edge of your bench with the table down (point up). Open the jaws of the gauge wider than the stone, lay the gauge flat on the bench, and gently close the jaws around the stone. You can see my index finger pushing against the edge of the bench while my thumb pulls the slider back.
Once you’ve picked up the stone, look at it from the side to be sure that it’s being held straight:
You can also use the sliding gauge to measure an inside diameter. There is a trick to this, though, having to do with the width of the jaw tips.
Since the tips of the jaws measure 5mm (1/2 cm) wide when closed, you have to ADD those 5mm to any reading you make. And, of course, you can’t measure any opening smaller than 5mm in diameter.
Open the gauge inside the object to be measured. Read the measurement as you would for an exterior measurement and ADD 5mm for the width of the jaws. The zero line on the Vernier scale is just a hair to the right of the 2mm mark on the metric scale, so we know the ring is at least 2mm + 5mm = 7mm in diameter. Looking closely at the short marks on the Vernier scale, we see that the second mark matches with the metric scale above, so we add .2mm to the measurement, giving us a final inner diameter of 7.2mm.
Do you have a digital caliper that’s a bit fussy about readings and chews through watch batteries? Never mind! Reach for your lovely, sturdy, simple sliding gauge instead, and get on with your work.
P.S. Did a friend forward this post to you? Did you stumble on it by accident? Want to eliminate the element of chance? Click here to get on my direct list.