Electrical resistance is measured by either of two methods: constant voltage or constant current.

The constant current technique sources a known current through an unknown resistance and the resulting voltage is measured. This technique is generally used for resistance values below 200M ohms. This approach is used most often by DMMs and other resistance measuring instrumentation. Examples of just some of the Keithley products which use this technique are 2000, 2001, 2002, 2010 and 2700.

The constant voltage technique sources a known voltage across an unknown resistance and measures the resulting current. This approach is used for high resistance (1e8 to 1e16) measurement applications. Resistance of this magnitude is normally a measurement of leakage, such as capacitor leakage, insulator resistance or relay contact isolation. The advantage of the constant voltage method lies in measuring the unknown resistance with various constant values of test voltage. This helps characterize the resistance with a parameter known as voltage coefficient, which is a measure of how the resistance reacts to various magnitudes of test voltage. Examples of just some of the Keithley products which can measure resistance with this method are Electrometers and SourceMeter Instruments.