A particular example of a Wheatstone bridge is the metre bridge.
The resistors 3 and 4 are replaced by a metre-long wire, and the galvanometer is connected to a movable contact. When the contact touches the
wire, the ratio of the lengths (l1 and l2) can be used in the following formula to find the resistance of the unknown resistor. It is easier to use (and understand) than the Wheatstone bridge. However, on the downside it is large and cumbersome.
METRE BRIDGE
R1 R2
l1 l2
R1 and R2 are the resistances of the two resistors and l1 and l2 are the length of the sections of the metre wire as illustrated in Fig 18.29
l1 l2
R1
R2
Fig 18.29
SAMPLE PROBLEM 18G
A metre bridge was used to measure the resistance of an unknown resistor R. Using the values in Fig 18.30, calculate the value of the unknown resistance, assuming the metre bridge was balanced.
15 R
62.3 cm
37.7 cm
Fig 18.30
SAMPLE ANSWER 18G
Using the formulae for the metre bridge, making sure the values of resistance are in their standard unit and the lengths both use the same unit:
