Some substances, such as carbon, conduct electricity fairly well but not really well. The conductivity can be changed by adding impurities like clay to a carbon paste, or by winding a thin wire into a coil. Electrical components made in this way are called resistors. They are important in electronic circuits because they allow for the control of current flow. The better a resistor conducts, the lower its resistance; the worse it conducts, the higher the resistance.

Electrical resistance is measured in units called ohms. The higher the value in ohms, the greater the resistance, and the more difficult it becomes for current to flow. For wires, the resistance is sometimes specified in terms of ohms per unit length (foot, meter, kilometer, or mile). In an electrical system, it is usually desirable to have as low a resistance, or ohmic value, as possible. This is because resistance converts electrical energy into heat.