Yeah, good question. My first attempts were with fluids: water and air. Water doesn't work very well because there's a lot of resistance to flow unless the pipes are large in diameter. It's not practical for anything but the simplest of circuits. So I went with air for a long time before I realized it had no chance, either - when it compresses and decompresses, it heats and cools, and that energy is lost to the surroundings. It makes horribly inefficient circuits. It's also hard to seal moving parts without adding an unacceptable amount of friction. And finally, you still can't see air moving through a circuit.

So I stepped back and thought about how to do it mechanically. But the hardest part of a mechanical circuit is the absolute simplest part in electronics: the junction. That is, where electricity flows in one wire and splits along two wires. How do you make chain or a belt split? Not only that, but it has to follow Kirchoff's law: the sum of currents leaving the junction must equal the current entering the junction. I finally realized that's what a differential gear arrangement does, and planetary gears are a sort of differential arrangement that would work perfectly for this. It was very, very hard to make the mechanical junction so that it had low resistance while under load, but I eventually got it. Once I had that, I knew it would all work.