This information applies to ink jet cartridges with the printhead in the cartridge like most HP and Lexmark cartridges. Most Epson ink jet printers use a different printhead technology.

The printhead and circuitry that perform most of the work of the ink jet printer are contained on the small ink jet cartridge itself. In the case of the HP 51626A high capacity cartridge, there are 48 nozzles or "jets" on the printhead located on the bottom of the cartridge.

Each nozzle or jet is smaller in size than a human hair and each is backed by a heater or resistor that heats and cools the ink inside the cartridge. When the ink is heated, a bubble forms. When the heat source is removed, the bubble "bursts" sending dots of ink on the page through the 48 nozzles. These dots form the print characters on the page at the rate of up to 6000 drops per second. The heating and cooling process is happening at very high rates of speed too.

It was once believed that the heaters in the cartridge would only last for one use of the cartridge. But we've learned that the heaters will continue to do their work and the jets will continue to fire for more than one use of the cartridge in most ink jet cartridges. In fact, the heaters or resistors on most ink jet cartridges will continue to fire until they either burn out completely or weaken significantly. On average, that happens from the third to fifth time the cartridge is filled.