Low Resistance Measurement Bridge

Below figure shows the schematic diagram of kelvin double bridge.

Low resistance measurement bridge.

But error occurs in the result when it is used for measuring the low resistance. The kelvin bridge is a variation of the wheatstone bridge which enables low resistances to be measured. Here r represents the contact resistance between the unknown resistance and the standard resistance whose effect we need to eliminate. Wheatstone bridge along with operational amplifier is used to measure the physical parameters like temperature strain light etc.

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. 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. Although today digital multimeters provide the simplest way to measure a resistance. 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.

A kelvin double bridge is a variant of the wheatstone bridge used for measuring very low resistances. The kelvin bridge is suitable for measuring the low resistance. A low resistance measurement is typically a measurement below 1 000 ohm. Sensitive null detector or galvanometer is required to detect balance condition.

This is the reason because of which the wheatstone bridge is modified and the kelvin bridge obtains. Wheatstone bridge use for measuring the resistance from a few ohms to several kilo ohms. The wheatstone bridge is used for measuring the very low resistance values precisely. The measurement range would typically be 1mω to 1kω with the smallest resolution of 1µω.

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. 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.