Null method in AC

Zeroing methods are widely used in metrology. They enable the quantity to be measured to be compared either directly, or via a measuring bridge, with a standard. The choice of one or other of these methods depends on the nature of the quantity to be measured, the standards available, the frequency at which the measurement is to be carried out and the desired uncertainty. In this article, we consider only impedance measurements.

The most commonly encountered and simplest bridges are Wheatstone-type bridges. They are therefore given special attention (§ 2 ). The various configurations in which they can be used are described, with the type of impedance for which they are best suited. Transformer bridges are also widely used, partly because of their good accuracy and the relatively low influence of environmental conditions on their performance, and partly because of their relatively low cost. The principle of these bridges is also presented (§ 3 ).

Since the achievement of equilibrium is a key point in all zeroing methods, the choice of the "zero detector" that enables this equilibrium to be achieved can become crucial. The most effective technique for extracting a low-amplitude alternating signal from noise that may be clearly dominant, known as the "synchronous detection technique", is described in detail, and the two main families of modern synchronous detectors currently available on the market are presented (§ 4