Abstract: According to a first aspect of the present invention there is provided apparatus arranged to determine an interface between two components, comprising: a reference electrode arranged to be connected to a constant voltage supply or ground; a measurement electrode, and a field generating device arranged to establish an electric field in at least one of the components; wherein the field generating device is configured to establish the electric field across the measurement electrode, and the apparatus is configured to measure a potential difference between the measurement electrode and the reference electrode, the potential difference between the measurement electrode and the reference electrode being indicative of the interface between the components.