INTRODUCTION
In this experiment, you will examine how to make simple DC measurements involving current,
voltage, and resistance.
The current I through, and the potential difference V across, a resistor of resistance R are related by
Ohm's Law:
V = IR
In MKS units, V is in volts (V), I in amperes (A), and R in ohms(). The power dissipated by a
resistor--often referred to as Joule heating—is
P = I2R = V2/R
Any network of resistors connected across a potential difference has the same behavior as a single
equivalent resistor with resistance we denote as R.
That is, the equivalent resistor draws the same current and dissipates the same power as the resistor
network.
There are several ways the equivalent resistance R can be determined. We describe two of them
here. First, as is evident from the figure above, it can be determined by measuring the potential
difference V across the network and the total current I drawn by the network and then calculating R
= V/I. Second, the equivalent resistance can be calculated from the individual resistances in the
network by successively replacing series and parallel combinations of resistors by their equivalent
resistors. Resistances in series and in parallel add to the equivalent resistance as shown below.
Series
R = R1 + R1 + R3 + …
Parallel
1
1
1
1



 ...
R
R1 R2
R3
Two or more resistors form a series combination if the current that flows through one is exactly the
same current that flows through each of the others in the combination. Two or more resistors form
a parallel combination if their respective ends are at the same potential. (It follows that the
potential difference across a resistor is the same for all resistors in a parallel combination.)
Name(s)______________________________________________________Section____________
DC CIRCUITS
A. Resistance measurements.
1. Use the resistor color code given at the end of this experiment to determine the nominal
resistance value labeled on each of the 3 resistors. Write down your results, and be sure to
remember which resistor is which.
2. Your multimeter can be used as a voltmeter, ammeter, or ohmmeter, depending on the switch
setting. Set the multimeter to be used as an ohmmeter and measure the resistance of each of the
resistors and compare with the color code values. When measuring resistance with an ohmmeter,
the resistance should be removed from the circuit, with no current flowing through the resistor
except from the ohmmeter. For ordinary resistors, the polarity of the ohmmeter is unimportant.
3. Now connect the series combination of the 3 resistors and measure the equivalent resistance.
Calculate the expected value using the series resistor formula, and compare with the measured
value.
4. Connect the parallel combination of the 3 resistors, measure the equivalent resistance, and
compare to the value expected from the formula.
B. Current and voltage measurements. When using an ammeter and a voltmeter, make sure that
they are connected as shown and that the proper polarity is observed. Specifically, the ammeter
must be connected in series with the part of the circuit for which the current is to be
measured. For the voltmeter, the complementary rule holds: it must be connected in parallel with
the part of the circuit for which the potential difference is to be measured.
ammeter
voltmeter
It is usually best to set the meter on a higher scale than you anticipate needing before making a
measurement, in order to avoid overloading the meter. You can then reduce the scale to obtain a
more sensitive reading if necessary.
Series circuit.
1. Wire the series circuit shown to the left. Measure the voltage across each
resistor and across the combination. Do the individual voltages add up to the
expected total?
2. Turn the power supply off, connect the multimeter so it is ready to
measure current, and have an instructor check your circuit before you turn the
power back on. Measure the current (the same for all resistors).
3. Use your measurements to compute the resistance of the series combination. Compare with the
ohmmeter measurements in part A. Calculate the total series combination, using the individual
resistance values measured with the ohmmeter, and compare with your measured effective
resistance.
Parallel circuit.
Now wire the parallel circuit shown below.
1. Turn the power supply off, and connect the multimeter so it is ready to measure the current
through R1. After an instructor has approved your circuit, turn the power on and measure the
current through each resistor and the total current from the power supply.
2. Measure the voltage drop (the same for all resistors).
3. Similarly, use your measurements to compute the resistance of the parallel combination, and
compare with the ohmmeter measurements in part A.
Calculate the total parallel combination, using the individual resistance values measured with
the ohmmeter, and compare with your measured effective resistance.
Series-parallel circuit.
Wire the circuit shown to the left, using any combination of resistors
that you wish to choose.
Measure all currents and voltages in the circuit. From your
measurements, compute the total effective resistance.
Calculate the total series-parallel combination, using the individual
resistance values measured with the ohmmeter, and compare with
your measured effective resistance.
QUESTIONS
1. Were Kirchoff's rules found to hold for the three circuits in part B?
2. An ideal voltmeter has an infinite resistance, and an ideal ammeter has zero resistance. The
voltmeter that you used in part B has a resistance of about 1 M. The ammeter has a resistance?
Measure !
Resistor Color Code
Color
black
brown
red
orange
yellow
green
blue
violet
gray
white
gold
silver
Value
0
1
2
3
4
5
6
7
8
9
+5%
+10%