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Rogowski coil
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<p>A Rogowski coil, named after <a href="/facts/Walter_Rogowski/KdU0HEba">Walter Rogowski</a>, is an electrical device for <a href="/facts/Electrical_measurements/10uAjN19">measuring</a> <a href="/facts/Alternating_current/qxICS3xa">alternating current</a> (AC) or high-speed current pulses. It sometimes consists of a <a href="/facts/Helix/BqC4AKwL">helical</a> coil of wire with the lead from one end returning through the centre of the coil to the other end so that both terminals are at the same end of the coil. This approach is sometimes referred to as a <i>counter-wound</i> Rogowski. </p><p>Other approaches use a full toroid geometry that has the advantage of a central excitation not exciting standing waves in the coil. The whole assembly is then wrapped around the straight <a href="/facts/Conductor_(material)/BSuyupLP">conductor</a> whose current is to be measured. There is no metal (iron) core. The winding density, the diameter of the coil and the rigidity of the winding are critical for preserving immunity to external fields and low sensitivity to the positioning of the measured conductor. </p><p>Since the <a href="/facts/Voltage/EN2po6N2">voltage</a> that is induced in the coil is proportional to the rate of change (<a href="/facts/Derivative/7Ito70Dh">derivative</a>) of current in the straight conductor, the output of the Rogowski coil is usually connected to an electrical (or electronic) <a href="/facts/Integrator/VTqXQuQU">integrator</a> circuit to provide an output signal that is proportional to the current. Single-chip signal processors with built-in analog to digital converters are often used for this purpose. If the ratio of the coil's inductance to its resistance (the RL time constant) is significantly greater than the length of the current pulse being measured, the coil is considered "self integrating". When both ends of the coil are connected together, the current in the coil is proportional to the current being measured. Connecting the ends of the coil together through a low-value resistor allows the current to be measured by measuring the voltage drop over the resistor. Thus, the device produces an output voltage proportional to the current being measured. </p>