Capacitive sensors are most often used to measure the change in position of a conductive target. But capacitive sensors can be effective in measuring presence, density, thickness, and location of non-conductors as well. Non-conductive materials like plastic have a different dielectric constant than air. The dielectric constant determines how a non-conductive material affects capacitance between two conductors. When a non-conductor is inserted between the probe and a stationary reference target, the sensing field passes through the material to the grounded target. The presence of the non-conductive material changes the dielectric and therefore changes the capacitance. The capacitance will change in relationship to the thickness or density of the material. For example this is applied in the nuclear industry to measure the thickness of oxide layers on nuclear rods. Non-conductors can me measured by passing the electric field through them to a stationary conductive target behind.