Transcript Document

Which is the correct way to measure current and voltage?
Good ammeters have low resistance
Good voltmeters high resistance
+
-
Use if low
resistance
A
V
+
-
Use if high
resistance
A
V
Using a Wheatstone Bridge means you don’t need an
accurate (expensive ) voltmeter or ammeter.
What will be the potential difference on the voltmeter?
12V
400
400
V
400
800
Wheatstone Bridge
15 k
12V
75 k
R1
R2
V
V
30 k
X k
R3
R1 R3

R2 R4
R4
When bridge is balanced ( meter
reads zero)
Tutorial Questions 25 to 27
Experiment
Find the resistance of A ,B and C
R1
R2
Discuss
V
R3
- choice of R1 and R2.
R4
R1 R3

R2 R4
- the resistor in series with the meter.
- why a shunt is used.
A is between 100  and 400 
B is over 10 k
C is less than 10 .
Tutorial questions 28 to 30
Using an out of balanced Wheatstone Bridge
V
1. Adjust the bridge so it is balanced at the required
temperature eg 20oC
V
2. Plot a graph of R and voltage.
3. Calibrate this graph to give temperature.
Tutorial questions up to and including 32
SAQs to page 27 end of booklet
0
R