Fig 18.27: Circuit diagram for a Wheatstone bridge
R4
R1 R2
R3 R4
R1, R2, R3 and R4 are the resistances of the four resistors as illustrated in Fig 18.275
Measuring resistance The Wheatstone bridge is used to measure the resistance of an unknown resistor.
Advantages: It is more accurate than an ohmmeter, multimeter or combination of ammeter and voltmeter.
Disadvantages: It is less compact or portable and it can take more time to set up than an ohmmeter,multimeter or combination of ammeter and voltmeter.
SAMPLE PROBLEM 18F In Fig 18.28, find the value of R4 if the bridge is balanced.
10 30
If no current is flowing then it shows B and C are at the same potential. When this happens,the Wheatstone bridge is said to be balanced.When it’s balanced, if three out of the four resistors are of known resistance, the fourth (unknown) resistance can be calculated using the formula:
7
R4
Fig 18.28
SAMPLE ANSWER 18F Using the formulae for the Wheatstone bridge, making sure all values are in their standard unit:
R1 R2
The value of R4 is 21 . 5The derivation of the formula is available on the website www.gillmacmillan.ie for illustrative purposes. 324 INVESTIGATING PHYSICS
