What is now commonly known as the Rogowski coil was first presented in 1887 by A.P. Chattock as a winding that was evenly placed on a flexible solid piece of India rubber with a constant cross-section . It was intended for the measurement of magnetic potential, and thus called magnetic potentiometer, but it was recognized from the start that the voltage at its output could be used to measure time-varying current when the winding forms a closed loop around a current-carrying conductor. In his succinct paper, A. P. Chattock covered apparently everything that was necessary: the theory of operation, a method of precision manufacturing by means of a lathe, and calibration by means of mechanically switched dc currents, long before the power electronics evolved and evaluated its measurement error which was about 1%. However, the widespread name of this device, Rogowski coil, became associated with W. Rogowski who co-authored a widely cited paper on the same device some twenty-five years later . The Chattock-Rogowski coil can be described as a conductive helical coil wound on a non-magnetic core encompassing the (primary) conductor carrying the current that is to be measured, as Fig. 1 shows. The coil output (secondary) is usually voltage, but it can also be current, in which case it is an air-core counterpart of the magnetic-core current transformer. For over one century after its inception, the essence of the Chattock-Rogowski coil remained the same but the technology of its manufacturing, the materials used, and the number of its applications have expanded tremendously . The Chattock- Rogowski coil is a very versatile device used primarily for the measurement of ac steady-state, impulse , transient , and pulsed  currents. The currents that can be measured range from a milliampere to over 10 MA, and operating frequencies range from fractions of a hertz to over a gigahertz. They are used extensively in the electric power industry by electric utilities an- electrical equipment manufacturers for switchgear, metering, and protective relaying  in substations, on-site calibrations of current transformers , and measurements for hydropower plant efficiency testing . They are also used in the steel industry for monitoring and control of electric arc furnaces, in the automotive industry for monitoring and control of current in resistance welding, in military applications, and in testing, calibration, and research laboratories, e.g., for measuring plasma currents in nuclear fusion research and measuring beam currents in particle accelerators.