d.c.-proof resistors are suited for permanent use in d.c. circuits. Normally, most resistor types are d.c.-proof. In the case of resistors where the conductor is embedded in hygroscopic compounds, alkaline or acid residues in the compounds or external precipitation combined with air moisture may result in the formation of an electrolyte. The d.c. will then cause ions to flow trough the electrolyte that will remove material from the negative pole of the resistor conductor, weaken it and finally result in fusion. This risk is particularly high in the case of high-ohm resistors with extremely thin resistor conductors while low-ohm resistors, particularly power resistors, are less affected because they are much thicker on the one hand and the resistor is often so hot during operation that any moisture will evaporate quickly, on the other.