Kelvin Bridge (Kelvin Double Bridge) – Working, Circuit, Advantages & Applications

Introduction

A Kelvin Bridge, also known as a Kelvin Double Bridge or Thomson Bridge, is a precise measuring instrument used to determine unknown electrical resistances that are less than one ohm. It is specifically designed for measuring four-terminal resistors with very low resistance values.

Need for Kelvin Bridge

Resistors with values greater than 1 ohm can be measured using instruments like an ohmmeter or a Wheatstone bridge, since the connecting wire resistance is negligible. However, for resistances less than one ohm, the resistance of connecting wires and terminals becomes significant and introduces measurement errors.

To overcome this issue, four-terminal resistors are used, particularly in precision resistors and high-current ammeter shunts. These resistors have two current terminals and two potential terminals. The current flows through the current terminals, while the voltage drop is measured across the potential terminals. This ensures that the measured voltage drop is due only to the resistor itself, excluding the parasitic resistance of connecting leads. A bridge circuit capable of accurately measuring such low resistances is known as the Kelvin Bridge.

Classification of Resistance

• High Resistance: Greater than 0.1 Mega-ohm
• Medium Resistance: 1 ohm to 0.1 Mega-ohm
• Low Resistance: Less than 1 ohm

Different instruments are required to measure each category of resistance. Methods such as the ammeter-voltmeter method or substitution method often yield large errors and are generally avoided for precise measurements. As resistance decreases, more accurate devices like the Kelvin Bridge are needed.

Limitations of the Wheatstone Bridge

While the Wheatstone Bridge is effective for measuring resistances from a few ohms to several mega-ohms, it produces significant errors when used for very low resistances due to the influence of lead and contact resistances.

Kelvin Bridge Circuit

When measuring low-value resistances with a Wheatstone Bridge, the lead and contact resistances increase the measured resistance value. In the Kelvin Bridge, this problem is minimized by a special circuit arrangement.

The resistance of the contacts connecting the unknown resistor (R) to the standard resistor (S) is represented by r.
Points m and n indicate the galvanometer connection points used to achieve a null balance.

If the galvanometer is connected to point m, the lead resistance r adds to the standard resistance S, giving a low indication of R. Conversely, if connected to point n, r adds to R, yielding a higher value. To eliminate this error, the galvanometer is connected at a midpoint d, where r is equally divided into r₁ and r₂. This ensures that the lead resistance does not affect the measurement.

Kelvin Double Bridge

The Kelvin Double Bridge is an advanced form of the Kelvin Bridge used for precise measurement of four-terminal low resistances. It incorporates two sets of ratio arms to eliminate the effects of connecting lead resistances.

The bridge structure is similar to the Wheatstone Bridge but includes additional connections. Ratio arms P and Q are connected to one end of the galvanometer, while another set p and q connects to the other end. A standard resistor S is placed between specific points to compensate for lead resistance effects.

When the ratio of P/Q equals p/q, the galvanometer shows zero deflection, and the resistance of the connecting leads has no influence on the measurement. This condition is known as the balance condition of the Kelvin Bridge.

Advantages of Kelvin Bridge

• Capable of measuring resistances from 0.1 Ω to 1.0 Ω
• Low power consumption
• Simple construction
• High sensitivity and accuracy

Disadvantages of Kelvin Bridge

• Requires a sensitive galvanometer to detect balance
• High current is needed for improved sensitivity
• Manual adjustment is required for balancing

Applications of Kelvin Bridge

• Used to measure unknown low resistances with high accuracy
• Commonly used in laboratories and industries for precise testing of conductor and shunt resistances

Conclusion

The Kelvin Bridge is an essential instrument for accurately measuring very low resistances where contact and lead resistances cannot be ignored. Its double-bridge configuration ensures precision and reliability, making it a valuable tool in electrical measurement and calibration systems.