Measurement of Low, Medium and High Resistance
Resistance measurement
1: Classification of resistance
 Low resistance is the range of 0.1ohm to 1 ohm.
 Medium resistance is the range of 1 ohm to low mega ohm.
 High resistance is 0.1 megaohm to a higher range.
2: Methods for the measurement of low resistance
 Kelvin double bridge
 Ammeter – voltmeter method
3: Methods for the measurement of medium resistance
 Ammeter – voltmeter method
 Wheat stone bridge method
 Ohm meter method
4: Methods for the measurement of high resistance
 Direct deflection methods
 Loss of charge methods
 Megha ohm bridge method
Kelvin double bridge for measurement of low resistance
Kelvin bridge, a modification of wheatstone bridge, method increases the accuracy in measurement of low resistance
and remove the effects of connecting leads and contact resistance. Here, r represents the resistance of connecting leads that connects the unknown resistance R and standard resistance S.
The galvanometer is connected at point d that divides the resistance r into r_{1} and r_{2 }such that,
$$\frac{r_{1}}{r{_{2}}} = \frac{P}{Q}$$
using equation we have,
$$ \Rightarrow \frac{r_{1}}{r_{1}+r_{2}} = \frac{P}{P+Q} \Rightarrow r_{1} = \frac{P}{P+Q} r $$
$$ \Rightarrow \frac{r_{1}+r_{2}}{r_{2}} = \frac{P+Q}{Q} \Rightarrow r_{2} = \frac{Q}{P+Q} r $$
from the figure at balance condition
$$\frac{R+r_{1}}{S+r_{2}}=\frac{P}{Q}$$
$$R+r_{1}= \frac{P}{Q} (S+r_{2})$$
$$R+\frac{p}{P+Q}r = \frac{P}{Q}(S+\frac{Q}{P+Q}r)$$
$$R=\frac{P}{Q}S$$
So, connecting the galvanometer at point d, the resistance of leads does not effect the result , but the problem with the above method are
 the method is not practical
 difficult to find correct galvanometer null point.
To solve the above problems, two actual resistance unit of correct ratio is connected between points m and n as shown in figure which is original kelvin double bridge, the ratio arm of p and q is connected at d to eliminate the effect of connection.
Under balance condition there is no current through Galvanometer, Thich means E_{ab }= E_{int,}
_{where,}
This equation shows that the resistance of connecting leads has no effects but error may be introduced in the ratio arms , ie. p/q = P/Q may not be equal. thermoelectric effect can be removed by reversing the battery connection and true value of R will be the mean of two readings.
In AV method the mesured resistance is given by R_{m }= V/I, where v and I is the voltage and current reading of voltmeter and ammeter respectively. The available connection methods are shown in figure.
AV measure to measure medium resistance
1: Classification of resistance
 Low resistance is the range of 0.1ohm to 1 ohm.
 Medium resistance is the range of 1 ohm to low mega ohm.
 High resistance is 0.1 megaohm to a higher range.
2: Methods for the measurement of low resistance
 Kelvin double bridge
 Ammeter – voltmeter method
3: Methods for the measurement of medium resistance
 Ammeter – voltmeter method
 Wheat stone bridge method
 Ohm meter method
4: Methods for the measurement of high resistance
 Direct deflection methods
 Loss of charge methods
 Megha ohm bridge method

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