Dielectric heating involves the heating of electrically insulating materials by dielectric loss. Electrolytic capacitors have a metallic anode covered with an oxidized layer used as dielectric. A capacitor consists of two conductors separated by an insulator, also known as a dielectric. This oxide layer acts as the dielectric of the capacitor. Capacitance hygrometers consist of two charged plates separated by a polymer membrane dielectric. Optical fibers are examples of "dielectric waveguides". BEOL processing involves creating metal interconnecting wires that are isolated by dielectric layers. The capacitance, being proportional to the dielectric constant, also exhibits this frequency behavior. The breakdown voltage is proportional to the thickness of the dielectric. The maximum energy is a function of dielectric volume, permittivity, and dielectric strength. Photonic crystals contain regularly repeating regions of high and low dielectric constant. An example of a paraelectric material of high dielectric constant is strontium titanate. In ceramic capacitors, this is caused by degradation of the dielectric. Dipole moment values can be obtained from measurement of the dielectric constant. Superacids are strong acids even in solvents of low dielectric constant.