Ohmmeter usage
PARTS AND MATERIALS
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Multimeter, digital or analog
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Assorted resistors (Radio Shack catalog #
271-312 is a 500-piece assortment)
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Rectifying diode (1N4001 or equivalent;
Radio Shack catalog # 276-1101)
-
Cadmium Sulphide photocell (Radio Shack
catalog # 276-1657)
-
Breadboard (Radio Shack catalog # 276-174
or equivalent)
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Jumper wires
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Paper
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Pencil
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Glass of water
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Table salt
This experiment describes how to measure the
electrical resistance of several objects. You need not
possess all items listed above in order to
effectively learn about resistance. Conversely, you need not
limit your experiments to these items. However, be sure to
never measure the resistance of any electrically
"live" object or circuit. In other words, do not attempt to
measure the resistance of a battery or any other source of
substantial voltage using a multimeter set to the resistance
("ohms") function. Failing to heed this warning will likely
result in meter damage and even personal injury.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
1, chapter 1: "Basic Concepts of Electricity"
Lessons In Electric Circuits, Volume
1, chapter 8: "DC Metering Circuits"
LEARNING OBJECTIVES
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Determination and comprehension of
"electrical continuity"
-
Determination and comprehension of
"electrically common points"
-
How to measure resistance
-
Characteristics of resistance: existing
between two points
-
Selection of proper meter range
-
Relative conductivity of various
components and materials
ILLUSTRATION
INSTRUCTIONS
Resistance is the measure of electrical
"friction" as electrons move through a conductor. It is
measured in the unit of the "Ohm," that unit symbolized by
the capital Greek letter omega (Ω).
Set your multimeter to the highest
resistance range available. The resistance function is
usually denoted by the unit symbol for resistance: the Greek
letter omega (Ω), or sometimes by the word "ohms." Touch the
two test probes of your meter together. When you do, the
meter should register 0 ohms of resistance. If you are using
an analog meter, you will notice the needle deflect
full-scale when the probes are touched together, and return
to its resting position when the probes are pulled apart.
The resistance scale on an analog multimeter is
reverse-printed from the other scales: zero resistance in
indicated at the far right-hand side of the scale, and
infinite resistance is indicated at the far left-hand side.
There should also be a small adjustment knob or "wheel" on
the analog multimeter to calibrate it for "zero" ohms of
resistance. Touch the test probes together and move this
adjustment until the needle exactly points to zero at the
right-hand end of the scale.
Although your multimeter is capable of
providing quantitative values of measured resistance, it is
also useful for qualitative tests of continuity:
whether or not there is a continuous electrical connection
from one point to another. You can, for instance, test the
continuity of a piece of wire by connecting the meter probes
to opposite ends of the wire and checking to see the the
needle moves full-scale. What would we say about a piece of
wire if the ohmmeter needle didn't move at all when the
probes were connected to opposite ends?
Digital multimeters set to the "resistance"
mode indicate non-continuity by displaying some
non-numerical indication on the display. Some models say "OL"
(Open-Loop), while others display dashed lines.
Use your meter to determine continuity
between the holes on a breadboard: a device used for
temporary construction of circuits, where component
terminals are inserted into holes on a plastic grid, metal
spring clips underneath each hole connecting certain holes
to others. Use small pieces of 22-gauge solid copper wire,
inserted into the holes of the breadboard, to connect the
meter to these spring clips so that you can test for
continuity:
An important concept in electricity, closely
related to electrical continuity, is that of points being
electrically common to each other. Electrically common
points are points of contact on a device or in a circuit
that have negligible (extremely small) resistance between
them. We could say, then, that points within a breadboard
column (vertical in the illustrations) are electrically
common to each other, because there is electrical
continuity between them. Conversely, breadboard points
within a row (horizontal in the illustrations) are not
electrically common, because there is no continuity between
them. Continuity describes what is between points of
contact, while commonality describes how the points
themselves relate to each other.
Like continuity, commonality is a
qualitative assessment, based on a relative comparison of
resistance between other points in a circuit. It is an
important concept to grasp, because there are certain facts
regarding voltage in relation to electrically common points
that are valuable in circuit analysis and troubleshooting,
the first one being that there will never be substantial
voltage dropped between points that are electrically common
to each other.
Select a 10,000 ohm (10 kΩ) resistor from
your parts assortment. This resistance value is indicated by
a series of color bands: Brown, Black, Orange, and then
another color representing the precision of the resistor,
Gold (+/- 5%) or Silver (+/- 10%). Some resistors have no
color for precision, which marks them as +/- 20%. Other
resistors use five color bands to denote their value and
precision, in which case the colors for a 10 kΩ resistor
will be Brown, Black, Black, Red, and a fifth color for
precision.
Connect the meter's test probes across the
resistor as such, and note its indication on the resistance
scale:
If the needle points very close to zero, you
need to select a lower resistance range on the meter, just
as you needed to select an appropriate voltage range when
reading the voltage of a battery.
If you are using a digital multimeter, you
should see a numerical figure close to 10 shown on the
display, with a small "k" symbol on the right-hand side
denoting the metric prefix for "kilo" (thousand). Some
digital meters are manually-ranged, and require appropriate
range selection just as the analog meter. If yours is like
this, experiment with different range switch positions and
see which one gives you the best indication.
Try reversing the test probe connections on
the resistor. Does this change the meter's indication at
all? What does this tell us about the resistance of a
resistor? What happens when you only touch one probe to the
resistor? What does this tell us about the nature of
resistance, and how it is measured? How does this compare
with voltage measurement, and what happened when we tried to
measure battery voltage by touching only one probe to the
battery?
When you touch the meter probes to the
resistor terminals, try not to touch both probe tips to your
fingers. If you do, you will be measuring the parallel
combination of the resistor and your own body, which will
tend to make the meter indication lower than it should be!
When measuring a 10 kΩ resistor, this error will be minimal,
but it may be more severe when measuring other values of
resistor.
You may safely measure the resistance of
your own body by holding one probe tip with the fingers of
one hand, and the other probe tip with the fingers of the
other hand. Note: be very careful with the probes, as
they are often sharpened to a needle-point. Hold the probe
tips along their length, not at the very points! You may
need to adjust the meter range again after measuring the 10
kΩ resistor, as your body resistance tends to be greater
than 10,000 ohms hand-to-hand. Try wetting your fingers with
water and re-measuring resistance with the meter. What
impact does this have on the indication? Try wetting your
fingers with saltwater prepared using the glass of
water and table salt, and re-measuring resistance. What
impact does this have on your body's resistance as measured
by the meter?
Resistance is the measure of friction to
electron flow through an object. The less resistance there
is between two points, the harder it is for electrons to
move (flow) between those two points. Given that electric
shock is caused by a large flow of electrons through a
person's body, and increased body resistance acts as a
safeguard by making it more difficult for electrons to flow
through us, what can we ascertain about electrical safety
from the resistance readings obtained with wet fingers? Does
water increase or decrease shock hazard to people?
Measure the resistance of a rectifying diode
with an analog meter. Try reversing the test probe
connections to the diode and re-measure resistance. What
strikes you as being remarkable about the diode, especially
in contrast to the resistor?
Take a piece of paper and draw a very heavy
black mark on it with a pencil (not a pen!). Measure
resistance on the black strip with your meter, placing the
probe tips at each end of the mark like this:
Move the probe tips closer together on the
black mark and note the change in resistance value. Does it
increase or decrease with decreased probe spacing? If the
results are inconsistent, you need to redraw the mark with
more and heavier pencil strokes, so that it is consistent in
its density. What does this teach you about resistance
versus length of a conductive material?
Connect your meter to the terminals of a
cadmium-sulphide (CdS) photocell and measure the change in
resistance created by differences in light exposure. Just as
with the light-emitting diode (LED) of the voltmeter
experiment, you may want to use alligator-clip jumper wires
to make connection with the component, leaving your hands
free to hold the photocell to a light source and/or change
meter ranges:
Experiment with measuring the resistance of
several different types of materials, just be sure not to
try measure anything that produces substantial voltage, like
a battery. Suggestions for materials to measure are: fabric,
plastic, wood, metal, clean water, dirty water, salt water,
glass, diamond (on a diamond ring or other piece of
jewelry), paper, rubber, and oil.
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