What is Kelvin Bridge?

The Kelvin Bridge also called the Kelvin Double Bridge used to a very low value of resistance. it is a modified form of Wheatstone Bridge and its name was given by William Thomson known as Lord Kelvin. it works on 4 arms principle and 2 of them are connected in series and parallel configuration of resistance. It helps to measure a very low resistance value of about 1 ohm. Let’s get started with What is Kelvin Bridge? – Definition & Kelvin Double Bridge.

Main Components of Kelvin Bridge Circuit

The Kelvin Brdige circuit comes with different components that are explained here

Known Resistance: it is denoted as Rs and its constant value
Resistance under test: It is denoted as Rx and it is resistance that has to be measured
Galvanometer: This is a device that is used to measure the electric current.
Power Source: It provides the required current to measure accurate resistance value.

What is the need of Kelvin Bridge?

Wheatstone bridge is used for calculating resistance from some ohms to several kilo ohms. However, an error exists in finding value when it is used for low resistance measurement. it is the reason there is a modification made in Wheatstone Bridge and the resultant circuit Kelvin Bridge. The Kelvin bridge is best to use for low-resistance measurement.

The resistor value of more than one ohm can be calculated with different methods like using an ohmmeter or with the use of a Wheatstone bridge. For this resistor, the resistance of connecting wires or terminals is ignored then the resistance value. For resistors that have a resistance less than ohm, the resistance value of connected wires or terminals is important, and conventional methods will add them in finding value

For minimizing these undesired resistances low value of the resistor accurate resists and high current ammeter shunts are made with 4 terminal resistors.

This resistance comes with the paint of current terminals and the paint of voltage terminals. During the process, current is passed between current terminals but voltage loss across the resistor is found at voltage terminals. The voltage loss measured will be due to the resistor since the parasitic resistance of leads carrying current to and from the resistor is not added in voltage circuits. For measuring these resistances needed bridge circuit made to operate with 4 terminals resistance. That circuit is called Kelvin Bridge.

Kelvin Bridge Working Principle

The working of Kelvin Bridge is like the Wheatstone Bridge but comes with two more resistors. The resistors R1 and R2 are connected with external voltage points of 4 terminal unknown resistor Rs and unknown resistor Rx. The resistors Rx, Rx, R1, and R2 are Wheatstone bridge resistance. In this circuit, the parasitic resistance of the upper part of Rx and low part of Rx is external to the voltage measuring portion of the bridge and so not added in the calculation.

But the connection between Rs and Rx is added in the voltage measuring part of the circuit and so can affect the accurate result. To minimize this effect 2nd pair of R′₁ and R′₂resistors make pairs of arms of the bridge and are connected with an internal voltage of Rs and Rx. So detector D is connected between junction R1 and R2 and junction of R′₁ and R′₂.

The balance equation of this bridge is given here

Rx/Rs=R2/R1+Rpar/Rs . (R’1/(R’1+R’2+Rpar)). (R2/R1-R’2/R’1)

In the bridge circuit, the ratio of R’1 to R’2 is configured with the same as the ratio of R1 to R2. In a result last term of this equation becomes 0 and we get.

Rx/Rs=R2/R1

Rearranging this equation we have

Rx=R2.Rs/R1

The parasitic resistance is removed from the balance equation and its existence does not affect finding the total result. This equation is like an equivalent Wheatstone bridge.

For practical uses, the magnitude of supply B can be configured to give current rough Rs and Rx or close to read operating current of the small rated resistor. it helps to small errors in measurement. This current does not pass through the measuring bridge.

Kelvin Double Bridge

In the circuit, we can check that

E=A/(A+B) x F

F=I x( C+D +((p+q)/p+q+t)xt)

So G the is voltage loss between a and = I x( C+((p+q)/p+q+t)

or A/(A+B) x I (C+D +((p+q)/p+q+t)x t)

= I (C+((p)/p+q+t)x t)

C= A/B x D+ (p)/p+q+t (P/Q-p/q)———– B

let suppose A/B=p/q then C= A/B x d

A/B=p/q

In this A/B=p/q equation, p and q are used to make a connection galvanometer at an accurate point between j and k to remove the influence of connection pins of electrical resistance t. The voltage loss between a and b is equal to F for the balance condition. for zero galvanometer reading E is equal to F.

Read Also:

What is Kelvin Bridge? – Definition & Kelvin Double Bridge

What is the need of Kelvin Bridge?