1. Wheatstone Bridge Circuits
Notes p.10
Wheatstone Bridge Circuits
A “Wheatstone Bridge” circuit is simply a parallel circuit with 2 branches. Each branch is a potential divider.
The “bridge” connects the branches as follows… R1 R2 R3 R4 Vs V

2. A Wheatstone Bridge is “balanced” when the voltmeter reads 0V (or ammeter reads 0A).
This happens when the resistor ratio on the top branch = the resistor ratio in the bottom branch.
R1 = R3
R R4

3. A Wheatstone Bridge Circuit Activity (Practical)
Aim: To find out the unknown value of the resistor Rx, using a Wheatstone
Bridge Circuit.
Method:
Circuit
V S R1 R2 A R3 Rx
2. Alter R3 until the ammeter reads 0 mA.
3. Note the value of R3 .

4. Calculation
The value of Rx can be calculated now using
Rx = R2
R3 R1

5. R3 (W) DR3 (W) Bridge current (mA) -800 -600 -400 -200 +200 +400 +600
Out of Balance Wheatstone Bridge
Starting with your bridge balanced, change the variable resistor by small amounts and record the reading on the mA meter.
R3 (W)
DR3 (W)
Bridge current (mA)
-800
-600
-400
-200
+200
+400
+600
+800

7. Out of Balance Graph
Bridge current (mA)
DR3 (W)

8. V a DR I a DR Out of Balance Conclusion
For small changes to one resistor, R, in a balanced Wheatstone Bridge, the out-of-balance voltage (or current) is directly proportional to the change in resistance.
V a DR
I a DR

9. The Wheatstone Bridge
Any method of measuring resistance using an ammeter and voltmeter involves some error unless the resistance of the meters are taken into account.
To remove these errors the meter would have to read zero.
A Wheatstone Bridge Circuit makes use of a zero reading (null deflection) to find an unknown resistor value.
Watch the demo of a metre bridge and complete the “Metre Bridge” sheet.

10. Quick Problems
Calculate the value of the unknown resistor that would balance the following Wheatstone Bridges.
a) R1 = ? , R2 = 10 W, R3 = 20 W, R4 = 80 W.
b) R1 = 50 W , R2 = ? , R3 = 50 W, R4 = 25 W.
c) R1 = 300W , R2 = ? , R3 = 200W, R4 = 170W.
d) R1 = 180W , R2 = 200W, R3 = ? , R4 = 400W.
e) R1 = 320W , R2 = 60W, R3 = 2kW, R4 = ? .
Problems: Continue with Problems up to 32.
Revision questions for Higher Physics,
page 53, Q. 1 – 7.

11. Quick Problems
Calculate the value of the unknown resistor that would balance the following Wheatstone Bridges.
a) R1 = 2.5 W , R2 = 10 W, R3 = 20 W, R4 = 80 W.
b) R1 = 50 W , R2 = 25 W , R3 = 50 W, R4 = 25 W.
c) R1 = 300W , R2 = 255W , R3 = 200W, R4 = 170W.
d) R1 = 180W , R2 = 200W, R3 = 360W , R4 = 400W.
e) R1 = 320W , R2 = 60W, R3 = 2kW, R4 = 375 W .
Problems: Continue with Problems p.11, Q