When I was a very young boy in my Midwestern small town, an older boy a couple of doors down, named David, was always showing me new and interesting tools. The first was a round magnifying glass. He concentrated sunlight and burned a hole through a piece of paper. I thought is was amazing. The second item I recall was a really tiny spirit level. I spent a lot of time checking how level the kitchen table was versus the kitchen cabinet, and other surfaces. I seemed so neat, and the color of the fluid was mesmerizing. I did not realize it had been specifically chosen:
The color of a spirit level vial is traditionally a range of yellow-green because, quite simply, this is the central range of color most visible to the human eye in all lighting conditions. On either side of the visible light spectrum is purple-blue on one side moving toward non-visible ultraviolet light and orange-red on the other moving toward the non-visible infrared light.
It seemed so simple, just center the bubble between the lines, and you knew it was level. As a boy I did not realize that a measurement like this only guaranteed something was level just along the line of the level. Levels have a 0 and 90 degree set of vials (tube that has the fluid and bubble with graduations). If you place this on a surface, and the bubbles in both vials are centered, the surface is level. This is useful for leveling RVs.
I vaguely recall either my father or grandfather (who had a huge level for his carpentry work) showing me how to check a level. You would just reverse it end-to-end and check to see if the bubble reading was the same.
Recently, my significant other wanted to adjust a number of picture frames that had become crooked. She asked me to eyeball them, which works, but I suggested just using my small bullet spirit level. Suddenly she was of the opinion this device should be in her care, rather than in my lab. I balked at the idea. My level has a strong magnet to hold it to metal surfaces. I ordered a couple of smaller, and somewhat inexpensive levels made from acrylic plastic. When they arrived, I was taken aback that the scale on them was in inches, but the level sensitivity is in millimeters per meter.
The sensitivity of a level tells you how sensitive the bubble in the vials is to a change from level. The bubble on a level with 1.0 mm/m sensitivity which is tilted, will move much more than a level with only 10.0 mm/m of sensitivity. Hultafors has a nice video demonstrating the difference using levels with these two sensitivities. Another way to look at this is that if you have a bubble centered, and the sensitivity is 1.0 mm/m then a line extended to one meter will be within +/- 1 mm of level. Of course in ‘merica, sensitivity is in inches per foot, because why use one compatible set of units when two are available.
Out of curiosity, I began to see if any levels are available with millimeter-only metric scales. In many cases levels don’t have a scale on them as they are used for leveling, and not linear measuring. I set my browser for Australia and searched for levels with millimeter rulers/scales. All I could find were levels whose total length is labeled in millimeters, but the scale on them is centimeters. Over and over I found this:
In some cases I found them labeled with inches and millimeters for total length, and inches and centimeter scales for linear measurement. The only device I found with a level and millimeters is for use in finding angles:
And it has an inch scale on it! What I did find, is that if I wanted to have a level with millimeters, the Japanese have this solution:
Quoting Monty Python, “it’s really not much of a cheese shop is it?” There was another type of converter offered, but it was really not any better. Why not just sell levels with millimeters on them?
If there is to be a scale that’s on the level, let it be millimeters.
If you liked this essay and wish to support the work of The Metric Maven, please visit his Patreon Page and contribute. Also purchase his books about the metric system:
The first book is titled: Our Crumbling Invisible Infrastructure. It is a succinct set of essays that explain why the absence of the metric system in the US is detrimental to our personal heath and our economy. These essays are separately available for free on my website, but the book has them all in one place in print. The book may be purchased from Amazon here.
The second book is titled The Dimensions of the Cosmos. It takes the metric prefixes from yotta to Yocto and uses each metric prefix to describe a metric world. The book has a considerable number of color images to compliment the prose. It has been receiving good reviews. I think would be a great reference for US science teachers. It has a considerable number of scientific factoids and anecdotes that I believe would be of considerable educational use. It is available from Amazon here.
The third book is called Death By A Thousand Cuts, A Secret History of the Metric System in The United States. This monograph explains how we have been unable to legally deal with weights and measures in the United States from George Washington, to our current day. This book is also available on Amazon here.