The Secret Power of the Sliding Gauge


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.

The unassuming Vernier scale bides its time below the cm.

The unassuming Vernier scale bides its time below the metric 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.

Here’s how:


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.


Hold gauge flat against bench to pick up stones.

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:

Stone girdle should be level in gauge jaws for most accurate measurement.

Stone girdle should be level in gauge jaws for most accurate measurement.

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.

Brass Gauge jaw tips - 5mm wide when closed.

Brass Gauge jaw tips – 5mm wide when closed.

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.

Measuring a ring ID (inner diameter)

Measuring a ring ID (inner 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.


Signature - Lowther

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32 Responses to The Secret Power of the Sliding Gauge

  1. JVBurnett says:

    This is an excellent explanation — thank you very much.

  2. Sharman Martin says:

    Hi Julia, Thank you so much for all your postings. Although I have heard or read about much of info you have discussed so far, what usually happens is I end up with more questions than answers. Your explanations are so clear and practical. You should write a book! I’d buy it, so you have already sold one 😉

    • julialowther says:

      Thank you, Sharman! I’m blushing a bit, and extremely gratified, since this is exactly how I hope my readers will feel. Confirmation from serious students like you means a lot to me :).

  3. Debbie Davis says:

    Thank-you sooooo much, I’ve stared at this thing so many times thinking I should really know how to read it, now I do!!! Thank-you

  4. Nisa says:

    Hi, Julia! What a great explanation! I do actually know how to use my little brass Vernier gauge (largely because I have to teach freshmen biology students to use the Vernier scale on their microscopes — works exactly the same way!), but I may borrow your explanation to help them out and to jog my memory when I pull the brass gauge out of my bench and go, “now, how does this work again??”
    Thanks for sharing!

  5. Pam Timm says:

    I heard about this vaguely a few months ago, but no one has posted a clearer explanation then what you’ve presented here! Thank you Julia 🙂

  6. AmieH says:

    Great explanation…Thanks!

  7. DianT says:

    I tossed one sliding gauge because I thought it was defective. Ordered replacement and got another defective one! I’ve kept that to use the side as a line drawing edge, and bought a digital caliper. Would you believe in 6 years, I’d never run across anything to explain that wonky scale? Heehee, learning gaps. Thank you so much!

  8. Brilliant! I’ve not been using mine correctly either. Thank you!

  9. Jerry says:

    Thank You it has been on my work bench a long time

  10. Laura says:

    Who knew- this is great information!

    • Jerry says:

      Thank You for opening my eyes to that gauge. I had to print that info and put it on my board.

  11. Pingback: Wire Gauges – Why They Are Useful | Shoebox Studio: Jewelry Making Techniques

  12. Rebecca says:

    This is an amazing explanation!! Thank you!! Finally someone who understands a lot of us jewelry artists struggle with measurements sometimes! I can move on lol!

  13. Pingback: Wire Gauges – Why They Are Useful | Shoebox Studio: Jewelry Making Techniques

  14. Claude Languirand says:

    Hi, I’ve had my brass calipers for upwards of 10 years. When I bought mine , I could buy the sister piece for depth measurement. I scored the back of each and epoxied them together; and they haven’t budged…And now…having an interest in jewelry making, you show me the what and where of this great tool.


  15. Linda K. says:

    I LOVE THIS!!! I had no idea that the sliding gauge could provide so much information. I’ve never even heard of the Vernier scale before. These gauges should come with instructions when you purchase them.

  16. Randy says:

    Love your informative tip!

  17. Lin says:

    Thank you for your clear explanation of this gauge. I never could figure it out, and I had never heard of the name Vernier Scale.

  18. ajdales says:

    Hi Julie, Nice explanation. However in addition to your story, modern steel sliding gauges have different thongs on the top side for measuring inside diameters. With these you can also measure smaller ID’s.

    Greetings Andre

  19. I am hoping that I may post a link to this page on my Facebook Professional. I am a metalsmith and for the first time in a long time I no long have to explain how to use this device. This is an excellent explanation & I appreciate the effort it took to put it together.
    Anne Mitchell
    FB – Anne Mitchell Metalworks

    • julialowther says:

      Hi Anne, Thank you for the compliments both here and on your Facebook page! I am delighted that you want to share this post with your Facebook readers. Please urge them to subscribe to my newsletter, if you think my other posts would be of benefit to them. Best, Julia

  20. Thanks for this excellent tutorial!

  21. Lynn says:

    Wow! this is great to know. I’ve been using mine for years too and never knew this. Thank you for explaining this.

  22. I’ll have to read this a few times, I’m a slow learner, but after my first read I cannot tell you how grateful I am!

  23. Cindy says:

    Wow, I wish someone had told me this years ago! Excellent thank you. I am now going to go read the rest go your blog posts!

  24. Kelly says:

    Thanks for the explanation. I didn’t know that I had no idea how the sliding scale worked and had never heard of the vernier scale.

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