Disclosed are packet sterilisers for sterilising packaged articles. Some such packet sterilisers comprise: a working surface arranged for receiving a packaged article to be sterilised a first electrode and a second electrode wherein the first electrode and the second electrode extend behind a portion of the area of the working surface and the first electrode is disposed between the second electrode and the working surface and the first electrode comprises a plurality of gaps arranged so that, in use, when a voltage difference is applied between the first electrode and the second electrode, the associated electric field is able to extend through the gaps beyond the working surface and into a package of said packaged article to be sterilised, wherein the first electrode and the second electrode comprise adjacent extended surfaces which lie substantially along the direction of the working surface that provides a capacitance related to the adjacent spatial extent of the first electrode and the second electrode and an inductance is provided, wherein the inductance is selected based on that spatial extent to modify the resonant frequency of the electrode arrangement.
