Why conventional wheatstone bridge cannot be used for measuring low resistance?
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HERE IS YOUR ANSWER....
Wheatstone bridge measures every type of impedance -high or low; real or imaginary.
Wheatstone bridge is the most general and fundamental bridge of difference type measurement in circuit. since it is difference and comparable type, it depend upon us that how low or how high resistance we desire to measure, on keeping following things in priority:
a) Other three resistances are known to appreciable precision;
b) Galvanometer are very accurate with high resolutions and
c) Minimum and shortest connecting wire is used to make the arrangement.
Considering these things, we can define a bridge sensitivity, SB and Galvanometer sensitivity Sv for voltage and Si for current.
Sv and Si are related as:
Sv = SiEquivalent bridge resistance + Galvanometer Resistance
Let us consider a Θ, the galvanometer deflection, then
SB = ΘδRR.
We can see no matter how small δR be, the chosen Galvanometer allows us to calculate the as small value as we are please at.
But in practical, so much care as of a), b) and c) are not taken care of. Because Galvanometer, a pmmc instrument, always suffers an iron loss and a hysteresis loss with time. Hence a compensation resistance, corresponding to every arm, is deployed to complement the effect of lead resistance and sensitivities, we use Kelvin double bridge method.
Do note that Kelvin bridge is not a different bridge, but the extension and family of Wheatstone bridge like Maxwell bridge, Wien bridge etc.
HERE IS YOUR ANSWER....
Wheatstone bridge measures every type of impedance -high or low; real or imaginary.
Wheatstone bridge is the most general and fundamental bridge of difference type measurement in circuit. since it is difference and comparable type, it depend upon us that how low or how high resistance we desire to measure, on keeping following things in priority:
a) Other three resistances are known to appreciable precision;
b) Galvanometer are very accurate with high resolutions and
c) Minimum and shortest connecting wire is used to make the arrangement.
Considering these things, we can define a bridge sensitivity, SB and Galvanometer sensitivity Sv for voltage and Si for current.
Sv and Si are related as:
Sv = SiEquivalent bridge resistance + Galvanometer Resistance
Let us consider a Θ, the galvanometer deflection, then
SB = ΘδRR.
We can see no matter how small δR be, the chosen Galvanometer allows us to calculate the as small value as we are please at.
But in practical, so much care as of a), b) and c) are not taken care of. Because Galvanometer, a pmmc instrument, always suffers an iron loss and a hysteresis loss with time. Hence a compensation resistance, corresponding to every arm, is deployed to complement the effect of lead resistance and sensitivities, we use Kelvin double bridge method.
Do note that Kelvin bridge is not a different bridge, but the extension and family of Wheatstone bridge like Maxwell bridge, Wien bridge etc.
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