Ohm is the unit used to measure resistance to the passage of an electric current. All materials resist the flow of electric current. But some materials offer more resistance than others. Materials that offer little resistance are called conductors. Materials that offer great resistance are called insulators. Electrical resistance, measured in ohms (R), is equal to the electromotive force producing a current, measured in volts (E), divided by the current, measured in amperes (I):ohms = volts/amperes or R = E/I