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A specialized version of the Wheatstone bridge network designed to eliminate, or greatly reduce, the effect of lead and contact resistance and thus permit accurate measurement of low resistance. The circuit shown in the illustration accomplishes this by effectively placing relatively high-resistance-ratio arms in series with the potential leads and contacts of the low-resistance standards and the unknown resistance. In this circuit RA and RB are the main ratio resistors, Ra and Rb the auxiliary ratio, Rx the unknown, Rs the standard, and Ry a heavy copper yoke of low resistance connected between the unknown and standard resistors.
As with the Wheatstone bridge, the Kelvin bridge for routine engineering measurements is constructed using both adjustable-ratio arms and adjustable standards. However, the ratio is usually continuously adjustable, over a short span, and the standard is adjustable in appropriate steps to cover the required range. See Bridge circuit, Resistance measurement, Wheatstone bridge