On Thu, 29 Nov 2018 20:30:11 -0000, "Bruce Farquhar"
wrote:

Those cheap Chinese water heaters which are simply two plates of metal, one on live and one on neutral, how can they work? Because the resistivity of tap water apparently varies from 2 to 200 ohm metres. That's a range of 100 fold in possible power output. I assume the resistivity changes due to impurities like lime in hard water areas?

It does not seem practical. what are the plates, I suppose AC would
limit electrochemical effects, but it does not sound practical.

As far as the resistance goes, I suppose with a mineral concentration
the resistance would fall with temperature, but at 100C the bubbles
would provide negative feedback.

Proof could be obtained by measuring the temperature. I would guess it
never goes over 100C :-)

AB