Low Resistance Measurement Bridge

This is the reason because of which the wheatstone bridge is modified and the kelvin bridge obtains.

Low resistance measurement bridge.

The limitations of the kelvin bridge are requires manual balancing. We can also measure the quantities capacitance inductance and impedance using the variations on the wheatstone bridge. A kelvin double bridge is a variant of the wheatstone bridge used for measuring very low resistances. A low resistance measurement is typically a measurement below 1 000 ohm.

The wheatstone bridge is used for measuring the very low resistance values precisely. Here r represents the contact resistance between the unknown resistance and the standard resistance whose effect we need to eliminate. But error occurs in the result when it is used for measuring the low resistance. R is the unknown low resistance and s is a standard resistance.

The measurement range would typically be 1mω to 1kω with the smallest resolution of 1µω. 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. The kelvin double bridge incorporates the idea of a second set of ratio arms hence the name double bridge has come and the use of four terminal resistors for low resistance arms. 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.

Although today digital multimeters provide the simplest way to measure a resistance. Wheatstone bridge along with operational amplifier is used to measure the physical parameters like temperature strain light etc. Below figure shows the schematic diagram of kelvin double bridge. The kelvin bridge is suitable 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. 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.