Kelvin double bridge

Here, the unknown resistance is compared with a known resistance using a suitable bridge. Resistance above I Q. can be measured by Wheatstone bridge. Resistance less than I Q can be measured by a Kelvin double bridge, where the lead resistance must also be compensated. 

The Kelvin double bridge is a more sophisticated variant used for the measurement of very low resistance such as ammeter shunts or short lengths of cable. This is also operated on D.C. In industrial terms the digital D.C. low-resistance instruments are more convenient although somewhat less accurate. 

When a derivation process is used to evaluate the result of a measurement, calculations are inevitable. Since computation may at some stage involve truncation and/or rounding, these numerical processes may provide an error in the result. This form of error is usually of significance only if precise measurements are being made, for example, in time-dependent measurements in which tolerances of 0.001% (or smaller) are commoa Another source of calculation error results from simplification of formulas. An example of this is the balance equation for a Kelvin double bridge (Fig. 31) used for the measurement of low resistance which is simplified from... 

Probably the circuit most widely used for many years for the measmement of resistance is that attributed to Sir Charles Wheatstone. Whereas he was an accomplished scientist, making many contribntions to science and engineering, he only evaluated the application of the bridge circuit that bears his name. The circuit, shown in its basic form in Fig. 5, was originally devised by S. H. Christy in 1833. This circuit has been nsed in various forms in a proliferation of applications from the Kelvin double bridge used for low-resistance measmement (Fig. 31) to the nn-balanced arrangement used in many transducers. 

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