how the thermostat of an electrical rice cooker works ?

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ranatungawk

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Hi;

Does any body knows how the thermostat of an electrical rice cooker works ? I found that there were only a spring and a magnet inside the sensor (see the pic). Basically I know that electrical rice-cooker works on the “water’s Latent heat” theory. How it sense 100C with only a magnet and a spring ? there were no any Bi-metal parts either …
 
The magnet could have it's Curie temperature at 100C. At that temperate the magnet losses it magnetism and the spring will open the switch. When it cools below the Curie temperature the magnetism returns and closes the switch.

Some soldering irons use that technique to control the tip temperature.
 
I'm surprised that such a device works but it does however very differently from how you stated cruschow.

If a permanent magnet is heated to the point where it loses it's magnetic properties it will not remagnetize when it cools. What's occuring here is the spring element itself is heated to it's currie point where it ceases to be magnetic, the magnetic field still exists in the permanent magnet, the material of the spring itself simple ceases to be affected by it above the currie point.

Here's a link to a commercial interrupter that uses the method.
 
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To be even more exact, at the Curie temperature the magnet goes from being magnetic to paramagnetic.
 
I'm sorry crutschow, what you're saying is completely and totally incorrect.

A permanent magnet heated to it's Currie point will lose it's magnetic properties when cooled bellow the curie point unless an externally applied magnetic field is imposed upon the material during the cooling process, this is the fundamental method used to make modern permanent magnets.

Here's a quote from the above article that's relevent.

Temperature

Temperature sensitivity varies, but when a magnet is heated to a temperature known as the Curie point, it loses all of its magnetism, even after cooling below that temperature.
Click to expand...
In the case of ranatungawks device the magnets Currie point is never reached, the spring's however is, so it cease to be affected by the magnetic field and it's spring force asserts itself.

The Wikipedia link to the Currie point states that the material becoming paramagnetic is reversible when it cools, however this has nothing whatsoever to do with the magnetic alignment of the material, just it's ability to be magnetic.
 
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Sceadwian you are correct. It is the spring (or a ferromagnetic material attached to the spring) that reaches its Currie temperature, not the magnet.
 
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A friend of mine owed a restaurant that used rice cookers. His were Panasonic.

 
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