Lab 2
31
Basic Measurement Concepts And Practices
Resistance
Summary
In 1827 G.S. Ohm, a German scientist, proved experimentally that the electrical current flowing
in certain materials (metals) resulted in a voltage drop across the element that was directly proportional to
the current. This relationship is known as Ohm’s Law.
Ohm’s Law is written as
V
=
IR
, where
V
is voltage measured in Volts,
I
is current measured in
amps and
R
is measured in ohms. This law is very important. It enables us to solve for the current flow in
an element if the voltage across it and its resistance are known. Or the voltage may be determined for if
both the current flow and the resistance are known. When the resistance of an element remains constant,
the voltage across the element plotted against the current through the element is a straight line with a
slope of
R
(Fig. 1). Such elements are called linear resistive circuit elements.
Every material exhibits resistance. Conductors such as metal wires and ionized gases have very
low resistance. Insulators such as air, glass wood and plastic, have resistances which are extremely high
compared to conductors. Semi conductors such as silicon and germanium have resistances between insu
lators and conductors. Super conductors have extremely low resistances. Resistance is measured in a
variety of ways. In this experiment you will use a digital multimeter and a power supply to measure the
resistance of different components.
"All that glistens may not be gold, but at least it contains free electrons.”
 John Desmond Bernal
English crystallographer and discoverer of Xray diffraction from protein crystals.
Figure 1: Current  voltage relationship for a resistor. When the voltage across a resistor
is plotted against current through it, the slope of the line gives the resistance.
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Lab 2
32
Educational Objectives
After performing this experiment, students should be able to:
1.
Measure resistance using the DMM.
2.
Use the function keys of the DMM.
3.
Program the DC power supply.
4.
Plot and interpret characteristic currentvoltage curves.
5.
Measure the characteristic currentvoltage curves of diodes.
6
. Measure the resistance of a thermistor as a function of temperature and voltage.
7
. Examine the resistivity of conductive foam strips.
Background Information
In this experiment the energy source is a programmable DC power supply. You should particu
larly read the description of the model you are using. When considering a resistor in a circuit, we use
Ohm’s law to determine the resistance of a linear element.
where
V
is voltage in Volts (V),
I
is current in Amperes (A) and
R
is in Ohms
(Ω
). Figure 2 depicts a
quick way of remembering how this law can be used to solve for the third quantity among
V
,
I
and
R
, once
the other two quantities are given.
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 Fall '10
 Cattell
 Resistance, Resistor, Electrical resistance, George Ohm

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