Kelvin bridge and kelvin double bridge

KELVIN BRIDGE
KAROLINEKERSIN E
ASST.PROF /BMIE
12/7/2020 1

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.
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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.
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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.
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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.
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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.
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• 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.
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• 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.
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• 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
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• 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.
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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.
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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

• 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
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• 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.
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, P/Q = p/q then above equation becomes
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• 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.
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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.
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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.
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APPLICATION
It is used to measure the unknown resistance of a wire.
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Kelvin bridge and kelvin double bridge

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