increase until the receipt of another feedback signal from the sensor when the antenna 70 receives a precursor signal this pattern is shown in fig1 as seen in fig1 the limit l at which the precursor signal occurs causing the controller 80 to reduce the operating voltage increases during the operation of the electrostatic atomizer accordingly the operating voltage applied to the charge injection device 21 is greater and the net charge injected into the stream of fluid 20 is therefore also greater than would be achieved by operating the charge injection device 21 on a constant voltage as the flow rate changes the atomizer takes into account the changing flow rate and obtains maximum net charge by increasing the charge until precursors occur in another embodiment of the invention the sensor includes an electrically isolated atomizer casing for detecting precursor signals in the vicinity of the orifice 22 in other embodiments the sensor comprises an electrode or some other probe in the vicinity of the orifice in other embodiments of the invention the electrostatic atomizer includes a charge injection device comprising an electron gun as disclosed in u s pat nos 5 478 266 5 391 958 5 378 957 and 5 093 602 hereby incorporated by reference herein the net charge would be varied by supplying the electron gun with a varying voltage as discussed above or by varying the.