1: Classification of resistance
2: Methods for the measurement of low resistance
3: Methods for the measurement of medium resistance
4: Methods for the measurement of high resistance
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
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
2: Methods for the measurement of low resistance
3: Methods for the measurement of medium resistance
4: Methods for the measurement of high resistance
No discussion on this note yet. Be first to comment on this note