This equation is the same as for the functionally equivalent Wheatstone bridge. In practical use the magnitude of the supply B, can be arranged to provide current through Rs and Rx at or close to the rated operating currents of the smaller rated resistor. This contributes to smaller errors in measurement. This current does not flow through the measuring bridge itself. This bridge can also be used to measure resistors of the more conventional two terminal design.

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This equation is the same as for the functionally equivalent Wheatstone bridge. In practical use the magnitude of the supply B, can be arranged to provide current through Rs and Rx at or close to the rated operating currents of the smaller rated resistor.

This contributes to smaller errors in measurement. This current does not flow through the measuring bridge itself. This bridge can also be used to measure resistors of the more conventional two terminal design. The bridge potential connections are merely connected as close to the resistor terminals as possible. Any measurement will then exclude all circuit resistance not within the two potential connections. Accuracy[ edit ] The accuracy of measurements made using this bridge are dependent on a number of factors.

The accuracy of the standard resistor Rs is of prime importance. As shown above, if the ratio is exactly the same, the error caused by the parasitic resistance Rpar is completely eliminated. In a practical bridge, the aim is to make this ratio as close as possible, but it is not possible to make it exactly the same. If the difference in ratio is small enough, then the last term of the balance equation above becomes small enough that it is negligible.

Measurement accuracy is also increased by setting the current flowing through Rs and Rx to be as large as the rating of those resistors allows. One such type is illustrated above. Laboratory bridges are usually constructed with high accuracy variable resistors in the two potential arms of the bridge and achieve accuracies suitable for calibrating standard resistors. For such use, the error introduced by the mis-match of the ratio in the two potential arms would mean that the presence of the parasitic resistance Rpar could have a significant impact on the very high accuracy required.

To minimise this problem, the current connections to the standard resistor Rx ; the sub-standard resistor Rs and the connection between them Rpar are designed to have as low a resistance as possible, and the connections both in the resistors and the bridge more resemble bus bars rather than wire. Some ohmmeters include Kelvin bridges in order to obtain large measurement ranges.

Instruments for measuring sub-ohm values are often referred to as low-resistance ohmmeters, milli-ohmmeters, micro-ohmmeters, etc.

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## Kelvin double bridge circuit for measurement of low resistance

In low resistance measurement, the resistance of the leads connecting the unknown resistance to the terminal of the bridge circuit may affect the measurement. Consider the circuit in Fig. The galvanometer can be connected either to point c or to point a. When it is connected to point a, the resistance Ry, of the connecting lead is added to the unknown resistance Rx, resulting in too high indication for Rx. When the connection is made to point c, R3, is added to the bridge arm R3 and resulting measurement of Rx is lower than the actual value, because now the actual value of R3 is higher than its nominal value by the resistance Ry. It is a Double bridge because it incorporates a second set of ratio arms.

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## Kelvin Bridge

It is the modified form of the Wheatstone Bridge. What is the need of Kelvin Bridge? Wheatstone bridge use for measuring the resistance from a few ohms to several kilo-ohms. But error occurs in the result when it is used for measuring the low resistance.

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## Kelvin bridge

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