a version of the standard-comparison method in which a signal that acts on a null indicator is proportional to the difference between a known quantity and the quantity being measured (the difference is reduced to zero). To reproduce physical quantities of a particular dimension for comparison, measures for the quantities are used in the null method.

Examples of the null method are the measurement of electrical quantities (electromotive force, voltage, capacity, resistance, and so on) and of nonelectrical quantities that are converted into electrical quantities (temperature, pressure, strains, and so on) by means of potentiometers and bridges. When using the null method with a four-arm bridge, the resistance of three of the bridge arms is adjusted in such a way that the current across the measuring diagonal of the bridge becomes zero. This condition, which corresponds to equilibrium of the bridge, is shown by a null indicator (usually a sensitive galvanometer) connected to the measuring diagonal. When the bridge is balanced, the value of the resistance being measured is determined from a known resistance in the comparison arm of the bridge and the ratio of the two other arms. The balancing of the comparison device can be made automatic.