COLLEGE OF ENGINEERING AND SCIENCE LABORATORY EXPERIMENT The Potentiometer Name:_________________________ ID:____________________________ Date:___________________________ Experiment-ThePotentiometer.doc Page 2 LABORATORY EXPERIMENT The Potentiometer Circuit Introduction One of the original purposes of the potentiometer was to measure an unknown voltage using a known voltage as a reference. The potentiometer comprises a series of precision resistors which can be placed in series with a precision slide-wire resistor to create a known resistance. The instrument is connected to known and unknown voltages and the resistors are adjusted until the voltages are in balance. It is used in conjunction with a galvanometer which indicates when zero current flows and that the variable resistance is correctly adjusted. One advantage of using a potentiometer circuit to determine the value of an unknown voltage source is that, when the potentiometer circuit is balanced, no current flows through the circuit which avoids inaccurate measurements due to internal resistances. More common applications of the potentiometer can be seen in the volume control on a radio or stereo, although this has been superseded by digital controls which click up or down. Today’s experiment expands on the techniques introduced in the laboratory, Measurement of Resistance, again using concepts given by Ohm’s and Kirchhoff’s laws. Aims Today’s laboratory aims to teach students how to connect a variety of electrical circuits in order to undertake reliable electrical measurements. This exercise is primarily a practical one which will: (a) provide experience in wiring up a potentiometer circuit, (b) use that circuit to measure unknown emfs, (c) use a potentiometer to calibrate an ammeter Reference Serway, R. A. and Beichner, R.J. (2000), Physics for Scientists and Engineers with Modern Physics, 5th ed., Saunders College Publishing, Ch. 28. Preliminary Questions The Concept of a Potentiometer 1. State the difference between the terms, electromotive force (emf) and terminal voltage, when referring to a battery? Include a reason for the numerical difference between the two in your answer and an equation relating the emf, , to the terminal voltage, V. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ .

1. COLLEGE OF ENGINEERING AND SCIENCE
LABORATORY EXPERIMENT

2. The Potentiometer
Name:_________________________
ID:____________________________
Date:___________________________
Experiment-ThePotentiometer.doc Page 2
LABORATORY EXPERIMENT
The Potentiometer Circuit
Introduction
One of the original purposes of the potentiometer was to
measure an unknown voltage
using a known voltage as a reference. The potentiometer
comprises a series of
precision resistors which can be placed in series with a
precision slide-wire resistor to
create a known resistance. The instrument is connected to
known and unknown
voltages and the resistors are adjusted until the voltages are in

3. balance. It is used in
conjunction with a galvanometer which indicates when zero
current flows and that the
variable resistance is correctly adjusted. One advantage of using
a potentiometer circuit
to determine the value of an unknown voltage source is that,
when the potentiometer
circuit is balanced, no current flows through the circuit which
avoids inaccurate
measurements due to internal resistances. More common
applications of the
potentiometer can be seen in the volume control on a radio or
stereo, although this has
been superseded by digital controls which click up or down.
Today’s experiment expands on the techniques introduced in the
laboratory,
Measurement of Resistance, again using concepts given by
Ohm’s and Kirchhoff’s laws.
Aims
Today’s laboratory aims to teach students how to connect a
variety of electrical circuits
in order to undertake reliable electrical measurements. This
exercise is primarily a
practical one which will:
(a) provide experience in wiring up a potentiometer circuit,
(b) use that circuit to measure unknown emfs,
(c) use a potentiometer to calibrate an ammeter
Reference
Serway, R. A. and Beichner, R.J. (2000), Physics for Scientists
and Engineers with
Modern Physics, 5th ed., Saunders College Publishing, Ch. 28.
Preliminary Questions

4. The Concept of a Potentiometer
1. State the difference between the terms, electromotive force
(emf) and terminal
voltage, when referring to a battery? Include a reason for the
numerical difference
between the two in your answer and an equation relating the
voltage, V.
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Experiment-ThePotentiometer.doc Page 3
2. Calculate the correction that must be made to a voltmeter of
coil resistance 3 kΩ,
with a reading of 1.5 V, when it is being used to measure the
emf of a cell of internal
resistance 10 Ω?

5. _____________________________________________________
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A potentiometer circuit is a circuit designed to measure the
potential difference
between two points without drawing any current from the circuit
containing
them. The potentiometer circuit is such a useful circuit that it is
often pre-packed into
a box which enables numerous refinements that make it easier to
use and increase
the accuracy of measurements. The ‘box’ is called a
potentiometer.
3. If the two points mentioned before are the two terminals of a
cell, what does the
potentiometer measure?
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In this experiment we are going to construct our own
potentiometer which resembles
a commercial unit. The circuit shown in figure 1 involving R1
and R2 includes resistive
decade boxes with a range of 0 Ω to 1110 Ω in 1 ohm steps. The
sum of R1 and R2 is
always kept equal to 1110 Ω while the values of R1 and R2 are
varied over their
allowed ranges and the value of V is noted. In this experiment,

6. if the internal
resistance of the voltmeter is high, the current through the
circuit will be
approximately constant.
R1
E
R2
A
C
B
V
Figure 1
4. State the relationship between the potential difference (PD)
between C and B and
the value of R2? Note: The current has a constant value.
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5. Why was the instruction to keep R1 + R2 = constant (1110 Ω)
included?

7. _____________________________________________________
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Experiment-ThePotentiometer.doc Page 4
6. Using a conventional definition, which of the two points, C
or B, has the higher
potential? Why?
7. State the range of PDs that can be tapped between C and B?
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8. Based on the decade boxes supplied from figure 1 determine
the smallest increment
of PD that can be tapped?
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In figure 2, the voltmeter (in figure 1) is replaced with a
battery, Ex and connected
according to the illustration. Note the relative polarities of E
and Ex.
R1

8. E
R2
A
C
B
Ex
I
Figure 2
9. Describe what will happen to the current through the battery,
Ex, if the PD between C
and B is adjusted to equal the PD of the battery Ex? Note: This
point, which is
normally recorded by noting the value of R2, is known as the
balance point.
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10. Determine the relationship between the PD of the battery
and its emf under these
conditions of balance.

9. _____________________________________________________
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11. State the relationship between Ex, the current drawn from
the driving battery, I, and
the value of R2 at balance, Rx? Note: The resistor sum is 1110
Ω.
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Experiment-ThePotentiometer.doc Page 5
12. Imagine the cell Ex is replaced with another cell, Es (an
accurately known standard
cell) and the value of R2 re-adjusted until balance is obtained,
changing the value of
R2 to Rs. How would you compare this outcome to the
equivalent relationship in
question 11?
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13. Write down Ex, therefore, in terms of Rx, Rs and Es?

10. _____________________________________________________
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Present all your answers to the demonstrator
before proceeding with this experiment
Procedure: Part 1
The circuit illustrated in figure 3 enables you to meet the
requirements of questions 9 to
13.
3 V
R1
R2 Ex
G
K1
K2
Es
Figure 3

11. K2 is a two-way switch that enables the connection of either Ex
or Es into the circuit. The
galvanometer, a sensitive device for measuring current, is
represented by G. The normal
position of K1, a tapping key, is open.
(a) Wire up the circuit shown in figure 3 and set R1 and R2
each to 555 Ω. Show your
circuit to the demonstrator before proceeding.
Caution
Handle the standard cell with care. Always keep it in an upright
position.
Never short its terminals. Never connect it directly to a
voltmeter. The
standard cell should only be connected into a circuit where its
emf is almost
balanced to prevent it from drawing any appreciable current.
Experiment-ThePotentiometer.doc Page 6
Always obtain balance points through the method of null
deflection, i.e. set R1
(and R2). Close key K1. If the galvanometer deflects, release
K1 immediately and
adjust R1 (and R2) and repeat until balance is obtained.

12. (b) Connect Ex and adjust R1 and R2, ensuring their sum
remains equal to 1110 Ω until a
balance point is obtained. This value of R1 is Rx. Record this
reading.
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(c) Connect Es and balance the circuit for Es, keeping the sum
of R1 and R2 equal to
1110 Ω. This value of R, is Rs. Record this reading.
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(d) Use the expression obtained in preliminary question 13 to
evaluate Ex, the emf of the
unknown cell.
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(e) Repeat steps (b) to (d) at least twice which will enable you
to estimate the
uncertainty in Ex.
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13. _____________________________________________________
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Note: It is likely that the tolerance of the resistors used to
construct the decade
boxes is 10%, and may result in a systematic error in your
calculations. Suggest a
way to test that tolerance if it is not provided on the box.
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Procedure: Part 2
The purpose of this section is to use the potentiometer to
calibrate an uncalibrated
ammeter, that is, to find a relationship between the correct
current flow through an
ammeter, I, and the reading on the ammeter Im. The basis for
this exercise is the
relationship outlined in question 1 of this part of the procedure.
Consider the circuit shown in figure 4.

14. Experiment-ThePotentiometer.doc Page 7
RH
3 V
R
x
y
Uncalibrated
ammeter
I
A
Figure 4
R is a standard resistor whose value is known to high-precision.
The function of RH, a
variable resistor, commonly called a rheostat, is to control the
current, I, through the
circuit.
Questions
1. State the relationship between the current, I, and the PD,
using the symbol Vxy,

15. across the standard resistor, R.
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2. Can this PD be measured accurately with a voltmeter? If not,
why not?
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(a) Connect the circuit shown in figure 4 to the circuit in figure
3 using the same 3 V
power supply; i.e. connect the points X and Y to replace the
unknown cell. This
enables the circuit in figure 3, the potentiometer, to determine
the PD across R. The
overall combined circuit is illustrated in figure 5.
RH
3 V
R = 170 W
x

16. y
Uncalibrated
ammeter
A
R1
R2 Es
G
K1
Rheostat
Figure 5
Experiment-ThePotentiometer.doc Page 8
(b) Take six measurements of Vxy using a variety of values of
Im in the range 1 to 10 mA
by adjusting the rheostat, which will enable you to calculate the
correct current
flowing through the ammeter circuit, I. Tabulate your results,
including uncertainties,
in the following table.

17. Meter reading Im
(mA)
Rx at balance
(Ω)
Vxy
(V)
Ixy
(mA)
Meter correction
Ixy - Im (mA)
Table 1
(c) Plot a meter correction graph, showing Ixy - Im on the
vertical axis and Im on the
horizontal axis.
(d) Write a brief conclusion to this laboratory session
explaining what a potentiometer is
useful for.
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18. _____________________________________________________
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Experiment-ThePotentiometer.doc Page 9