Bridges in Electrical Circuits

Measurement of Resistance

DC Bridges 

DC bridge plays a significant role in the measurement of resistance. In these two circuit branches are bridged by a third branch connected between the first two branches at some intermediate point along with them.

Wheatstone Bridge

The best-known bridge circuit, the Wheatstone bridge, was invented by Samuel Hunter Christie and popularized by Charles Wheatstone and is used for finding the value of unknown resistance. Wheatstone bridge is constructed from four resistors, two of known values R1 and R3 (see diagram), one whose resistance is to be determined Rx, and the variable R2.

Wheatstone Bridge

At the point of balance,

Unknown Resistance

The Wheatstone bridge has also been generalized to measure impedance in AC circuits.

Limitations of Wheatstone Bridge

The Wheatstone bridge gives inaccurate readings if it is unbalanced. The Wheatstone bridge measures resistance from a few ohms to megohms. The upper range of the bridge can be increased with the help of the applied emf, and the lower range is limited by connecting the lead at the binding post.

Kelvin Double Bridge

Kelvin Bridge is just like a Wheatstone Bridge with a few modifications. Primarily the modifications contain sets of arms, one with resistances P and Q and the other with resistances p and q. R is the unknown low resistance and S is a standard resistance. Here r represents the contact resistance between the unknown resistance and the standard resistance, whose effect we need to eliminate.

Kelvin Bridge

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. Hence for a balanced bridge, we can write:

Calculation for unknown resistance

Hence, we can eliminate the contact resistance completely and hence error due to it.