# Kelvin bridge and kelvin double bridge

Assistant professor um Avinashilingam university
7. Dec 2020
1 von 21

### Kelvin bridge and kelvin double bridge

• 2. KELVIN BRIDGE • The Kelvin bridge or Thompson bridge is used for measuring the unknown resistances having a value less than 1Ω. • It is the modified form of the Wheatstone Bridge. 12/7/2020 2
• 3. NEED OF KELVIN BRIDGE • Wheatstone bridge use for measuring the resistance from a few ohms to several kilo-ohms. • But error occurs in the result when it is used for measuring the low resistance. • This is the reason because of which the Wheatstone bridge is modified, and the Kelvin bridge obtains. The Kelvin bridge is suitable for measuring the low resistance. 12/7/2020 3
• 4. ELECTRICAL RESISTANCE • High Resistance: Resistance that is greater than 0.1 Mega-ohm. • Medium Resistance: Resistance that ranges from 1 ohm to 0.1 Mega-ohm. • Low Resistance: Under this category resistance value is lower than 1 ohm. 12/7/2020 4
• 5. PRINCIPLE OF KELVIN BRIDGE • A Wheatstone bridge is used to measure resistance equal to or greater than 1 – ohm, but if we want to measure the resistance below 1 – ohm, it becomes difficult because the leads which are connected to the galvanometer adds up the resistance of the device along with the resistance of leads leading to variation in the measurement of the actual value of resistance. • Hence in order to overcome this problem, we can use a modified bridge called kelvin bridge. 12/7/2020 5
• 6. MODIFICATION OF WHEATSTONE BRIDGE • In Wheatstone Bridge, while measuring the low- value resistance, the resistance of their lead and contacts increases the resistance of their total measured value. 12/7/2020 6
• 7. • The r is the resistance of the contacts that connect the unknown resistance R to the standard resistance S. • The ‘m’ and ‘n’ show the range between which the galvanometer is connected for obtaining a null point. 12/7/2020 7
• 8. • When the galvanometer is connected to point ‘m’, the lead resistance r is added to the standard resistance S. • Thereby the very low indication obtains for unknown resistance R. • And if the galvanometer is connected to point n then the r adds to the R, and hence the high value of unknown resistance is obtained. • Thus, at point n and m either very high or very low value of unknown resistance is obtained. 12/7/2020 8
• 9. • So, instead of connecting the galvanometer from point, m and n we chose any intermediate point say d where the resistance of lead r is divided into two equal parts, i.e., r1 and r2 The presence of r1 causes no error in the measurement of unknown resistance 12/7/2020 9
• 10. • The above equation shows that if the galvanometer connects at point d then the resistance of lead will not affect their results. • The above mention process is practically not possible to implement. For obtaining the desired result, the actual resistance of exact ratio connects between the point m and n and the galvanometer connects at the junction of the resistor. 12/7/2020 10
• 11. KELVIN DOUBLE BRIDGE • A kelvin bridge or kelvin double bridge is a modified version of the Wheatstone bridge, which can measure resistance values in the range between 1 to 0.00001 ohms with high accuracy. • It is named because it uses another set of ratio arms and a galvanometer to measure the unknown resistance value. • The basic operation of the Kelvin double bridge can be understood from the basic construction and operation of the kelvin bridge. 12/7/2020 11
• 12. KELVIN DOUBLE BRIDGE CIRCUIT • The ratio of the arms p and q are used to connect the galvanometer at the right place between the point j and k. • The j and k reduce the effect of connecting lead. The P and Q is the first ratio of the arm and p and q is the second arm ratio. 12/7/2020 12
• 13. • The galvanometer is connected between the arms p and q at a point d. • The point d places at the center of the resistance r between the point m and n for removing the effect of the connecting lead resistance which is placed between the unknown resistance R and standard resistance 12/7/2020 13
• 14. • The ratio of p/q is made equal to the P/Q. Under balance condition zero current flows through the galvanometer. • The potential difference between the point a and b is equivalent to the voltage drop between the points Eamd. 12/7/2020 14
• 16. , P/Q = p/q then above equation becomes 12/7/2020 16
• 17. • The above equation is the working equations of the Kelvins bridge. The equation shows that the result obtains from the Kelvin double bridge is free from the impact of the connecting lead resistance. • For obtaining the appropriate result, it is very essentials that the ratio of their arms is equal. The unequal arm ratio causes the error in the result. Also, the value of resistance r should be kept minimum for obtaining the exact result. • The thermo-electric EMF induces in the bridge during the reading. This effect can be reduced by measuring the resistance with the reverse battery connection. The real value of the resistance obtains by takings the means of the two. 12/7/2020 17
• 18. ADVANTAGES It can measure the resistance value in the range of 0.1 µA to 1.0 A. Power consumption is less Simple in construction Sensitivity is high. 12/7/2020 18
• 19. DISADVANTAGES For knowing whether the bridge is balanced or no, the sensitive galvanometer is used. To obtain good sensitivity of the device, a high current is required. Manual adjustments are to made periodically when required. 12/7/2020 19
• 20. APPLICATION It is used to measure the unknown resistance of a wire. 12/7/2020 20