My mom's pendulum clock

Can you recommend a source that explains about ems and expanding
text? Osmosis is good learning, I guess, but without the book, one
can miss things. Like my mother's antique pendulum clock that I
disassembled when I was 11 years old, to figure out how it worked. I
ended up with an extra wooden gear and the clock ran about an hour in
a minute, complete with chimes.

Come to think about it, that may be why she took out a contract on my
pet buzzard Stinky. I never quite forgave her for that, but maybe I
drove her to it.