Liquid level controls

I am putting together a talk on liquid level controls (LLCs). The basic outline will be an overview of LLCs through the ages, ending with some of the latest technologies and what's on the horizon. Here's my basic outline...

Early LLC's
float valves (Phillips, etc)

Later LLC's
float switches

Current LLC's
capacitance probes, optical sensors

New & future
Danfoss wire

The talk is not intended to promote one company or another. It's for RETA education.

My question(s)...

Are there any other LLC's I should be including?

Do I have the chronological order correct?

I'm most fuzzy on technologies before float switches and float valves (if there were any). And if I'm missing any of the new or "on the horizon" technologies. Again, not trying to promote any one technology over the other.

Websites and pictures are a plus. I've been trying to find a picture of an old Magnetrol mercury bulb switch.

Thanks in advance for your help.