in the typical embodiment of the invention shown in fig1 - 3 , a carriage 10 carrying an ink jet head 11 is moved in a reciprocating path indicated by the arrow 12 by a belt 13 which is driven by a drive motor 14 and is carried over a spindle 15 at the opposite end of the path . a control system 16 supplies signals through a line 17 to the drive motor 14 and also supplies signals through a cable 18 to the ink jet head 11 to control the ejection of ink drops 19 from the head toward a closely adjacent substrate 20 . the substrate 20 is driven by a drive roll 21 in a direction perpendicular to the direction of reciprocating motion of the ink jet head and the ink ejection control signals and the substrate drive signals are arranged so that the ink ejected from the head produces an image 22 on the substrate 20 . the ink jet head 11 is connected to lines in the cable 18 by a series of contacts 23 , one of which is shown in fig2 which engage corresponding contacts ( not shown ) in the carriage 10 . an ink reservoir 24 in the ink jet head 11 contains a supply of ink 25 which flows through a series of conduits 26 to a corresponding series of pressure chambers 27 . if hot melt ink is used in the head , appropriate heaters ( not shown ) are arranged to heat the ink and maintain it in a molten condition during operation , but permit the ink to be solidified when the system is not in use . each of the pressure chambers 27 is associated in the conventional manner with one of a series of orifices 28 through which ink is ejected when a pressure chamber transducer 29 receives an appropriate signal on a line 30 connected to the contact 23 . in the illustrated embodiment shown in fig2 the series of orifices 28 is arranged in a line disposed at an angle to the direction of reciprocating motion 12 of the head so that , during each pass of the head adjacent to the substrate 20 , printing is effected by ink ejected from selected orifices 28 to produce dots at appropriate places in a path consisting of a series of adjacent lines on the substrate 20 , after which the substrate is advanced by the drive roll 21 by a distance equal to the width of that path . in accordance with the invention , the embodiment of the ink jet head shown in fig2 includes a nonvolatile memory 31 and a pulse counter 32 which counts a series of pulses corresponding to the pulses supplied on the line 30 to cause ejection of ink drops through the orifices 28 . preferably , the nonvolatile memory 31 is preset at the time the reservoir 24 is filled with ink 25 at the factory with a number corresponding to the total number of drops intended to be ejected before the ink 25 in the reservoir 24 should be replenished , while leaving a predetermined quantity of ink to permit completion of , for example , one page of text , without using all of the ink in the reservoir . during operation , the pulse counter 32 supplies pulses to the memory 31 to deduct the total number of ink drops ejected from the head from the number retained in the memory and , when the number in the memory 31 reaches zero , a signal is provided by the memory to initiate appropriate action . such appropriate action may , for example , include immediate termination of printing , or transmittal of a warning signal to the control unit 16 , or setting of a latch which will prohibit feeding of another sheet of substrate 20 into the printer after completion of the printing of the sheet then being printed . fig3 illustrates the manner in which the usage counter responds to the signals supplied on the line 30 so as to control the number retained in the nonvolatile memory . in the example shown in fig3 a series of transducer control signals to be supplied to a series of 16 orifices is indicated by low and high signal levels 33 and 34 , the high signal levels 34 representing transducer actuation . each high - level control signal 34 initiates a corresponding usage counter control signal 35 which is detected by the counter 32 and transmitted to the nonvolatile memory 31 to be deducted from the number retained in the memory . after the number in the nonvolatile memory 31 has been reduced to zero and any additional printing has been completed , the replaceable ink jet head 11 is removed from the carriage 10 and replaced by another ink jet head with a factory - filled reservoir 24 . the used ink jet head may then be discarded or returned to a factory for refilling . if hot melt ink is used in the head , the total quantity of ink remaining in the head can be determined , for example , by weighing the head . the reservoir 24 is then replenished to provide a quantity of ink equal to the original quantity of ink and the nonvolatile memory 31 is reset at the factory to the original count . to prevent unauthorized replenishment of the reservoir , which could lead to cumulative errors in the counting of ink drops , a special code is required to reset the nonvolatile memory . in the embodiment of the invention described with reference to fig1 - 3 , the ink jet head 11 includes an integral counter 32 and nonvolatile memory 31 in which a number corresponding to the remaining quantity of ink in the reservoir is stored . consequently , the head can be removed from the carriage 10 and stored or moved to another ink jet printer without losing information regarding the quantity of ink remaining in the reservoir . in an alternative embodiment shown in fig4 protection against unauthorized refilling of an ink jet head 36 is provided by incorporating a fuse 37 in the head . in this case , the control system 16 , which is similar to the control system 16 shown in fig1 includes a system control unit 38 , a carriage drive 39 , and a printing control unit 40 , from which signals are supplied to the ink jet head through a line 18 to control ejection of ink drops . in this embodiment , however , the printing control unit 40 includes a counter 41 and a nonvolatile memory 42 for counting and retaining information about the number of drops ejected by the ink jet head , thereby reducing the cost and weight of the ink jet head 36 in comparison with the ink jet head 11 . whenever printing is initiated with the embodiment shown in fig4 the printing control unit 40 senses for the presence of a fuse 37 in the ink jet head 36 and , if the fuse is detected , the memory 42 is reset to a number corresponding to the full capacity of the ink reservoir in the ink jet head and the fuse 37 is blown . during operation , the counter 41 counts the number of drops ejected by the ink jet head and deducts that number from the number stored in the memory 42 and thereafter , whenever printing is initiated , the control unit 40 senses the absence of a fuse in the head 36 and does not reset the memory 42 . when the total number of drops ejected by the ink jet head reaches the number initially stored in the memory 42 , the same action described above with respect to the head 11 is taken , i . e ., the printing may be stopped immediately or a warning signal may be provided to the operator to replace the head or printing may continue until an appropriate time such as completion of the page then being printed , and then the printing operation is stopped until the head has been replaced . if the ink jet head 11 utilizes a polarizable material such as lead - zirconium - titanate ( pzt ) in the transducer which is actuated to eject drops of ink , the fuse 37 may conveniently consist of a selected portion of the transducer material in which a pattern of polarized and nonpolarized areas is stored when the reservoir in the ink jet head is filled . when the head is installed , the system detects the presence of the polarized pattern and , after resetting the counter , depolarizes the polarized portions of the pattern . although the invention has been described herein with reference to specific embodiments , many modifications and variations therein will readily occur to those skilled in the art . accordingly , all such variations and modifications are included within the intended scope of the invention .