A liquid drug is tested by creating an electric corona discharge in the vicinity of a generally wedge-shaped layer of the drug, and recording the image thereof on a photographic film. The film is placed on the top side of an insulation plate, and the drug is disposed between the film and a convex end of a displaceable electrode. The wedge-shaped layer of the drug is created by pressing the electrode against the film. A high frequency generator is connected to an electrode disposed on the bottom side of the plate to create the corona discharge.