I worked a lot with torque wrenches in the Army, and always wondered how the “first” torque wrench was calibrated without another one to verify that it was accurate? Was there another tool to verify the calibration was correct and if so how did that one get calibrated. In my head it keeps asking “well how did the next one get calibrated?” Every time I think about the first precision tools of any type, not just for torque.
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If you go back far enough, you just find that the units are arbitrary and were invented by a person or committee. Originally those “master” measurements were kept in the most precise way they knew how, such as with a platinum weight for the kilogram, or rod of particular length for the meter. These measurement standards have been replaced by observable physics, for instance the meter is now defined as the distance light travels in 1/299,792,458 seconds. This is easy to replicate with the right precision instruments, but is still arbitrary. If the original meter had been defined as twice as long, the modern definition would be the distance light travels in 2/299,792,458 seconds.
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