Re: Grease pencils and rheostats
One of my many non-study ways of filling my time at Oxford was doing light shows for balls*. For a big occasion we'd hire a van-load of kit from Strand Electric. The dimmer units were armour-plated monsters on wheels - typically about five feet long by four feet high to provide twelve 500W channels.
Then smaller dimmers using a solid-state device called a thyristor started to appear. I built a number of these, but mine always burned out. I guess I was destined for a career in software rather than hardware.
* Dances in marquees, not testicle lighting.