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): 
The resistance of a conductor depends on its dimensions and its temperature as well as on the material from which the conductor is made. For example, the resistance of a wire increases as its length increases or as its diameter decreases. Generally, the resistance of a metal increases as its temperature rises.
The international standard for the ohm was adopted in 1990. It was based on a sensitive measurement of the resistance of a conducting material in a magnetic field. When cooled to extremely low temperatures, the material’s resistance can have only certain, precise values. Engineers have worked out the resistance of standard-sized wires for people who work with electric circuits. Resistance in an electric circuit can be measured with an ohmmeter. More accurate measurements can be obtained by using a device called a Wheatstone bridge (see Wheatstone bridge ).
