Percolator

A percolator consists of a pot with a small chamber at the bottom, which is placed closest to the heat source. A vertical tube leads from this chamber to the top of the percolator. Just below the upper end of this tube is a perforated chamber.

The percolator is prepared for use by placing the desired quantity of water in the pot, and a corresponding amount of a fairly coarse grind of coffee in the top chamber. It is important that the water level be below the bottom of the coffee chamber.

After the percolator is placed on the heat source (such as a range or stove), the temperature rises until the water in the bottom chamber boils. While some models may feature a one-way valve at the bottom of the tube which forces some of the boiling water up the tube, most operate on the simple principle of rising bubbles forcing liquid up the tube. The hot water is distributed at the top over the perforated lid of the coffee chamber. This water then seeps through the coffee grounds and leaves the coffee chamber through the bottom, dropping back into the lower half of the pot. The rest of the colder water at the bottom is meanwhile also forced up the tube, causing this whole cycle to repeat continually.

As the brew continually drips through the grounds, the overall temperature of the liquid approaches boiling point, at which stage the "perking" action (the characteristic spurting sound the pot makes) stops, and the coffee is ready for drinking. In a manual percolator it is important to remove or reduce the heat at this point, as most coffee-drinkers agree that it should never be allowed to boil; an adage to this effect states, "Coffee boiled is coffee spoiled". Brewed coffee left on high heat for too long will also acquire a bitter taste.

Some coffee percolators have an integral electric heating element, and should obviously never be used on a stove. Most of these automatically reduce the heat at the end of the brewing phase, keeping the coffee at drinking temperature but not boiling.

Information Source: Wikipedia and Image Source: Cefs Catalog