in the representative embodiments disclosed in the specification , an ink jet system includes a carriage for driving an ink jet head having a reservoir containing ink in a reciprocating motion and a counter for counting ink drops ejected through orifices in the ink jet head , and a nonvolatile memory for storing a number corresponding to the number of drops ejected and for generating a signal to terminate operation of the head or to produce a warning signal or to permit completion of printing of a sheet being printed when the number in the memory reaches a selected value . the ink jet head , which is removable from the system , has a refillable reservoir and , to prevent unauthorized refilling , a security arrangement utilizing a special code or a fuse is provided .