Ed Purcell developed a set of constants/numbers that one could use to estimate physical quantities quickly. These notes could fit on a postcard and could be used for “back of the envelope” calculations. I obtained a copy of these notes from my Caltech PHY 103b Order of Magnitude class from 1999 (notes from 1997 here and the modern version is now called PHY 101:OOM). The gallery shows the card which should be downloadable and the back size of the card is a imperial/metric grid which can be useful for aligning and sizing laser beams.
The grad TA for the course was Sanjoy Mahajan, who has written some great books (all of them available open source electronically – see below – but as of 2025-05-27 seems to have disappeared from the internets?):
- Street Fighting Mathematics -> direct access to free Open access page
- The Art of Insight in Science and Engineering -> direct access to free Open access page
- His PhD dissertation, which was the notes for the course mentioned above: https://thesis.library.caltech.edu/5338/
-JTS
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