A drop-on-demand printer having a row of ink ejection locations (2) for ejecting plural ink droplets, each ejection location (2) having an associated ejection electrode (40) to which a voltage is applied for causing electrostatic ejection of the droplets from the respective ejection location (2); a guard channel (5) disposed between adjacent ejection locations (2), each guard channel (5) having an electrode (50) disposed therein; and control means for applying a voltage to said guard channel electrodes (50), said applied voltage being the average of the voltages applied in operation over a given time to the adjacent ejection location electrodes.
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