Abstract: In high frequency apparatus such as for example an rf resonant cavity or a waveguide, it is often necessary to include a slot to permit a probe or other member to be located in the high frequency region and be moveable along the slot. Sealing means arranged adjacent to the slot and around the probe permits movement of the probe whilst minimising the open aperture area of the slot, hence preventing or reducing leakage through it. In one embodiment of the invention, the sealing means comprises a metallic mesh and in another it includes an elastomer material which is loaded with metallic material and/or carries a metallic mesh.