The resistive heating element is simply a coiled wire, very similar to the filament of a light bulb or the element in an electric toaster that gets hot when you run electricity through it. In a resistive element like this, the coil is embedded in a plaster to make it more rugged. The heating element has two jobs:
- When you first put the water in the coffee maker, the heating element heats it.
- Once the coffee is made, the heating element keeps the coffee warm.
In the picture above, you can see how the resistive heating element is sandwiched between the warming plate and the aluminum water tube. The resistive heating element presses directly against the underside of the warming plate, and white, heat-conductive grease makes sure the heat transfers efficiently. This grease, by the way, is extremely messy (very hard to get off of your fingers!). You find this grease in all sorts of devices, including stereo amplifiers, power supplies -- pretty much anything that has to dissipate heat.
The coffee maker's switch turns power to the heating element on and off. To keep the heating element from overheating, there are also components such as sensors and fuses. In coffee makers, sensors detect if the coil is getting too hot and cut off the current. Then, when it cools down, they turn the current back on. By cycling on and off like this, they keep the coil at an even temperature. Fuses simply cut the power if they sense too high a temperature. They're there for safety reasons, in the event that the main sensor fails.
Another important part of the coffee maker is the one-way valve. This valve is usually either in the hole in the bucket or in the aluminum heating pipe. If there were no one-way valve, then the boiling water would be just as likely to flow back into the bucket as to rise up the white tube. The one-way valve lets cold water into the aluminum tube, but forces the bubbles of boiling water to flow up the white tube. If you blow on the tube leading into this value, the valve should be open. If you inhale through the tube, the one-way valve should block any air.
Now that we've got the basic elements down, let's turn on the coffee maker and take it for a test drive.