Chapter 27 Electrical Measurements M. L. Sanderson 27.1 Units and standards of electrical measUrement 27.1.1 si electrical Units The ampere (A) is the Si base unit (Goldman and Bell, 1982; Bailey, 1982). The Ninth General Conference of Weights and Measures (CGPM), in 1948, adopted the definition of the ampere as that constant current that, if maintained in two straight, parallel conductors of infinite length, of negli- gible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 # 10 -7 newton per meter of length. The force/unit length, force of 1 V is produced when the electric current varies uniformly at the rate of 1 A/s. The weber (Wb), the unit of magnetic flux, is the flux which linking a circuit of one turn would produce in it an electromotive force of 1 V if it were reduced to zero at a uniform rate in 1 s. The tesla (T) is a flux density of 1 Wb/m 2 . 27.1.2 realization of the si Base Unit The definition of the Si ampere does not provide a suitable "recipe" for its physical realization. The realization of the