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The switch will either be a monstrous bakelite affair embedded in a large ceramic trip circuit, where flipping it is like throwing a fire alarm lever and a large light will glow red just above the switch to indicate that water is being vapourised.
Or the switch will look surreptitiously like the light switch right next to it and the next time you are done urinating you will hear your dad scream approximately 15 minutes later: "Dai!! Why do you always leave the geyser switch on!!"
Why is this contraption called a Geyser? Well, because each time the water comes to a boil and the whole mechanism shudders and shakes against its restraints bolted to the wall with 1-inch lugnuts, one thinks of Old Faithful or how Iceland gets all its power from Geothermal.
Journalist: "So this single huge switch here controls the heavy water valves for the entire system? Tell our readers, is that really safe?"
Nuclear Scientist at Kalpakkam: "No, no. That is the geyser switch for our morning baths. This undhervun switch is the valve release. Very safe..."