<iframe width="560" height="315" src="https://www.youtube.com/embed/gNRnrn5DE58" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe> This video is great, definitely worth the half hour break. ([youtube link](https://youtu.be/gNRnrn5DE58)) - To achieve precision you need to compare things. To compare things you need a common reference frame; you need [flatness](https://en.wikipedia.org/wiki/Flatness_(manufacturing)). In the 1830s a method of achieving flatness was invented, laboriously grinding three plates against each other. Apparently flatness is a sufficient foundation for bootstrapping your way up every other shape you might want to manufacture precisely. - Precision used to be expensive! In order to determine the length of the meter France paid two experts to spend 6 years measuring the length of a single line across France. The first set of gauge blocks were worth $21k in today's money. During WWI gauge blocks were to hard to find they needed to be smuggled into the US. - I particularly loved the old clip showing off gauge blocks ([14:43](https://youtu.be/gNRnrn5DE58?t=885)). They are so precise that briefly warming one up with your hand will noticeably change their size! - I dig this description of the precision bootstrapping process. Much of the modern world relies on precision and the interchangeability it enables. Standardization happened piecemeal and started in little islands before slowly expanding across the world. First precision is expensive, then precision is cheaper, and eventually these islands unify and precision parts are competitive and cheap! - An earlier standard (1824) specified the meter in terms of how long a pendulum had to be in order to have a period of one second. We eventually realized that gravity is not uniform everywhere and needed a new standard. - My reading list now has two more books: - [Henry Ford - Moving Forward](http://gutenberg.net.au/ebooks17/1700321h.html) - [Foundations of Mechanical Accurary](https://archive.org/details/FoundationsOfMechanicalAccuracy/page/n31/mode/2up) - He mentions the chain of standard reference materials, though he places it inside the "web of precision", which strongly reminds me of the [[The Cosmic Distance Ladder]]! - "Dimensional metrology" is a neat word