My assumption was that these common kitchen appliances had the same mechanism for knowing when the toast / water / rice is at the right temperature or level of doneness.

Then I checked–and had a big surprise. Really?

Today’s SearchResearch Challenge is simple:

1. So… how DO each of these devices know when the toast / water / rice is ready?

2. (extra credit) What other devices do you believe you understand, but when you checked, you learned that you actually didn’t understand? Does anything spring to mind? Any surprises?

Searching for the answer isn’t hard:

[ how does an electric tea kettle know when
the water is boiling ]

[ how does a rice cooker know when
the rice is done ]

[ how does a toaster know when
the toast is done ]

Let’s talk about these one-by-one.

Electric tea kettle: Do this query and you’ll find a lot of explanatory text telling you that a ring-shaped bimetallic strip changes shape as it warms up, and since it’s slightly buckled, when it deforms just enough, it snaps into a different shape, mechanically flipping the heater switch to off when the water hits the boiling point of 100C (212F). Very simple, very clever. But how does it know?

The underside of a tea kettle with the bimetallic ring outlined with a red dashed line.

That bimetallic ring is actually a fairly smart little invention–the tang coming out of the ring lies in the plane of the ring at room temperature, but as the ring starts to bend under the heating of the kettle, it suddenly snaps to a different shape with the tang pointing up. This is called a bistable device, a gizmo that can be in one of two different physical configurations. Of course, the bimetallic strip’s clever trick is that it will flip into the “hot” state when the temperature gets high enough, then flip back to the cool state after the heat is off. That rapid flip to the other bistable position is what mechanically flips the power switch back off.

The best example of this I could find is this YouTube short showing a tea kettle thermostat that flips back and forth rapidly when warmed up. (Video link.)

And… I thought that was that.

But as I was writing this up, I ran across a video about electric tea kettles on Steve Mould’s YouTube channel that showed me something I’d completely missed: the steam tube!

(Steve Mould video short on steam tubes in tea kettles.)

It turns out (as Steve Mould points out), that the thermostat is actually at the bottom of the tube that is open to the top of the vessel. As the kettle heats up, eventually the steam in the kettle is forced down the tube where it directly heats up the thermostat, which flips at around 95 C (203 F).

Why is this important? Because water boils at different temperatures at different altitudes, so a thermostat that is preset to 100C won’t switch off at high altitudes. In Denver, which is at 1609 meters (5280 feet), water boils at 95°C, or 203°F. So if the thermostat only tripped at 100°C, it wouldn’t work!

At this point I got really curious about this somewhat sophisticated design, so I looked up a few electric tea kettle patents, where I learned that this is well-known by tea kettle designers. To quote one such patent (US4357520, from 1979):

“Alternatively a steam tube or passage, which communicates with a steam or vapour aperture in the upper wall of the container, may be run down the outside of the container. Such a tube or passage may be concealed within or behind a handle structure of the container…”

I thought that was it. NOW I understood how they worked.

But as I was writing this up, on a lark, I searched for:

[ electric tea kettle diagram ]

hoping to find a cross-section of a kettle showing the steam tube.

But NO! I was again surprised to learn that there is a second thermostat, a “boil dry” thermostat that kicks in if/when the first thermostat fails to detect any steam. This would happen if the kettle was switched on without any water. No water, no steam, no steam pressure, no thermostat clicking off.

I found this because I saw this diagram in the search results:

Electric tea kettle circuit diagram. (P/C from Karisimby’s blog)

I know enough about electronics to realize that this was NOT what I had in my head. The “steam thermostat” was what I expected. What I did not expect was a second “Boil dry” thermostat.

A bit more searching revealed that such a second thermostat is set at a much higher temperature and is attached directly to the kettle wall. So if the kettle is dry and heating up, the steam thermostat won’t kick in, but the boil dry thermostat will open the circuit and stop the heating from running away (and possibly causing a meltdown).

Amazing what you’ll learn if you keep poking. (If you want to learn more, I recommend this wonderful explanation of tea kettles.)

Rice cooker: Again, not a difficult query, but the answer surprised me! I’d assumed that there was a simple bimetallic thermostat in the rice cooker as well–in fact, I’d assumed that it worked in much the same way, probably with an identical part inside.

But I quickly learned that most rice cookers (especially older ones) use a very clever magnet that turns off the heating element when the temperature goes above 100°C, or 223°F.

Basically, there are two heaters in a rice cooker–one that keeps the rice warm and a second heater that boils the water, much like the tea kettle.

However, I learned that rice cookers have a little magnet that holds the switch closed to close the main circuit to the heating element. That circuit heats up the water until it boils, after it boils, the temperature of the magnent will start to rise beyond 100°C, or 223°F. When that happens, the magnet loses it’s magnetism, allowing the circuit breaker to pop open, stopping the heating.

The amazing trick here is that the magnet is designed so that it’s Curie point (that is, the temperature at which it loses its magnetism) is at 100°C!

This is simpler to watch a video than to explain in text. So I recommend this video: Nice explanation of how the magnetic cutoff switch works on a rice cooker.

The bimetallic strip thermostat works because different metals expand at different rates, causing it to change shape.

Oddly, I can’t find any evidence that a rice cooker has a boil dry mechanism! (I assume they do. If you find out, let me know in the comments below.)