Low Resistance Measurement Bridge

A kelvin double bridge is a variant of the wheatstone bridge used for measuring very low resistances.

Low resistance measurement bridge.

We can also measure the quantities capacitance inductance and impedance using the variations on the wheatstone bridge. This is the reason because of which the wheatstone bridge is modified and the kelvin bridge obtains. The limitations of the kelvin bridge are requires manual balancing. The wheatstone bridge was originally developed by charles wheatstone to measure unknown resistance values and as a means of calibrating measuring instruments voltmeters ammeters etc by the use of a long resistive slide wire.

Wheatstone bridge use for measuring the resistance from a few ohms to several kilo ohms. R is the unknown low resistance and s is a standard resistance. Its additional complexity over the basic wheatstone design is necessary for avoiding errors otherwise incurred by stray resistances along the current path between the low resistance standard and the resistance being measured. The kelvin bridge is a variation of the wheatstone bridge which enables low resistances to be measured.

The measurement range would typically be 1mω to 1kω with the smallest resolution of 1µω. Sensitive null detector or galvanometer is required to detect balance condition. A low resistance measurement is typically a measurement below 1 000 ohm. But error occurs in the result when it is used for measuring the low resistance.

For measurement we make the ratio p q equal to p q and hence a balanced wheatstone bridge is formed leading to null deflection in the galvanometer. The second set of ratio arms p and q is used to connect the galvanometer to a point d at the appropriate potential between points m and n to eliminate the effect of connecting lead of resistance r between the known resistance r and the standard. Wheatstone bridge along with operational amplifier is used to measure the physical parameters like temperature strain light etc. Although today digital multimeters provide the simplest way to measure a resistance.

Here r represents the contact resistance between the unknown resistance and the standard resistance whose effect we need to eliminate. At this level it is important to use test equipment that will minimize errors introduced by the test lead resistance and or contact resistance between the probe and the material being tested. Below figure shows the schematic diagram of kelvin double bridge.