After a few weeks of wrangling about in the lab, I was finally able to get some good data out of my experiment.
I haven’t fully analyzed the data yet, but my back-of-the-envelope calculations seem to indicate that I have experimentally measured the speed of light within about 6% of the accepted value.
Not bad for an experiment consisting of a laser, a few mirrors (with one strapped to an electric motor), a multimeter, and what basically amounts to a very fine ruler attached to a microscope. The distance the beam traveled was about 15 meters.
With better instruments, and a longer distance, and more beer it’s quite likely that I could refine this number even more.