A Kelvin double bridge is used to measure an unknown electrical resistance below 1 Ω. Its operation is similar to the Wheatstone bridge except for the presence of additional resistors. These additional low value resistors and the internal configuration of the bridge are arranged to substantially reduce measurement errors introduced by voltage drops in the high current (low resistance) arm of the bridge. It consist of two ratio arms. The outer ratio arm contains the known resistors and the inner ratio arm helps to connect the one terminal of the Null detector at the appropriate point (which was the disadvantage of the Kelvin first bridge).
The trainer kit contains an I.C regulated +5V @500 mA power supply, digital null detector, Kelvin double bridge, Decade resistances and set of low value unknown resistances .