Electricity passing through any material encounters an opposing force that is similar to mechanical friction. This opposing force in a conductor is called resistance. The value of the resistance is determined by the conductor size and material. In direct current (DC) applications resistance limits the length of the cable because of the voltage reduction caused by resistance. In digital applications, resistance contributes to the characteristic impedance and is the primary cause of voltage loss (attenuation). Cable resistance is specified in Ohms per foot (Ω/ft).