The improvement of electrohydrodynamic devices is hindered by the absence of necessary information on the functional dependence of injection current density on the electric field strength. The investigation of the issue presents a great challenge because the quantity measured experimentally is the total current that is dependent on some almost inseparable factors. Moreover, the present theoretical dependences contradict each other and have not been verified by the experiment. In view of this, the determination of injection function is an actual problem, and the paper is an attempt to solve it. The approach proposed here is based on a comparison of the so-called dynamical current-voltage characteristics, resulting from the experiment and computer simulation where a test injection function is used as a boundary condition and changed iteratively. The correctness of the final injection function was confirmed by good agreement of the velocity distributions obtained from computer simulation and the particle image velocimetry experiment.