1 Ripka, P. Electric Current Sensors: a Review Preprint Measurement Science and Technology. 2010, 21(11), 112001-112024. Electric current sensors: a review Pavel Ripka, Czech Technical University in Prague, Czech Republic Abstract The paper makes a brief overview of traditional methods of measuremet of electric current and shows in more detail relatively new types of current sensors. These include Hall sensors with field concentrators, AMR current sensors, magnetooptical and superconducting current sensors. The influence of the magnetic core properties on the error of the current transformer shows why nanocrystalline materials are so advantageous for this application. Built-in CMOS current sensor are important tools for monitoring of the health of integrated circuits. Of special industrial value are current clamps which can be installed without breaking the measured conductor. Parameters of current sensors are also discussed, including geometrical selectivity. This parameter specific for current sensors means the ability to suppress the influence of currents external to the sensor (including the position of return conductor) and also suppress the influence on the position of the measured conductor with respect to the current. Key words Hall current sensor, current transformer, AMR current sensor, current shunt, ammeter, Rogowski coil, current comparator, CMOS current sensor, magnetooptical current sensor, SQUID current sensor, GMI, current clamps 1. Introduction Precise DC and AC current sensors are required by the automotive industry and by the chemical industry for measuring power and energy, and in many other applications. We discuss the principles of these devices with special reference to recent advances. We will concentrate on factors influencing precision, geometrical selectivity and other parameters important for new applications, e.g. inverters for power mills, intelligent buildings and big technological or physical experiments, e.g. electron beam measurement in the ATLAS experiment at CERN. Classical current sensors were reviewed in [Iwansson 1999]. Most contactless current sensors are based on magnetic field sensors – these were reviewed in [Ripka 2001], and more recently in [Edelstein 2007]. The latest review of current sensors provides an excellent overview of the principles and properties of industrial devices available on the market [Ziegler 2009a]. In the present paper we concentrate on the latest research developments and laboratory devices. 2. Overview of current measurement 2.1 Definition and standards Electric current is caused by movement of charge, and in the older MKS system it was therefore naturally defined as a derived unit. In the present SI unit system, the ampere is the fundamental unit [BIPM]. The early (1908) definition of an ampere was electrolytic. This was replaced in 1948 by a force definition. Using this definition, the ampere can be realized using the current balance. The most precise definition is the Watt current balance, which is also used to realize the “electronic kilogram”: a relative uncertainty of 3.6×10 −8 was achieved by the U.S. National Institute of Standards and Technology (NIST) [Steiner 2007]. A much more practical realization of the ampere is via Ohm's law from the volt and the ohm,