Inkjet Printers - Response Times of Resistance Heater Elements in Inkjet Printers
The ink drops that make up the 'image' on the paper from an inkjet printer are expelled by microscopic resistance (thin film or the like) heaters with response times in the 10s of microseconds if I recall correctly. When living in the macro world, it is often counterintuitive to realize that a resistance element can have such a fast thermal response.
(From: John Eaton (johne@vcd.hp.com).)
The trick is that a lot of the energy that you pump into the resistor leaves the printhead with the fired dot. One way to detect Out_of_Ink is to mount a thermistor on the printhead and watch for a sudden rise in temperature as you are firing.