Understanding Electrical Resistance Testing

 
Testing the electrical resistivity of different materials
 
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Whether you are a scientist researching a new medicine or
an engineer solving climate change, safety always comes
first. An adult must always be around and supervising when
doing this activity. You are responsible for:
ensuring that any equipment used for this activity is in good
working condition
behaving sensibly and following any safety instructions so
as not to hurt or injure yourself or others
Please note that in the absence of any negligence or other
breach of duty by us, this activity is carried out at your own
risk. It is important to take extra care at the stages marked with
this symbol: 
 
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Imagine water flowing through
two pipes…
If one pipe has a narrow
section this means that less
water can flow through it.
The narrow pipe "resists" the
flow of water.
 
Less Resistance
 
More Resistance
 
Resistance to the flow of
electricity can be measured
using an ohmmeter
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This is an electrical
instrument capable of
measuring 
voltage, current
,
and resistance
 
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Test probes
 
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Mulitimeter – digital/analogue
 
Assorted resistors
 
Crocodile clips
 
Paper/pencil
 
Breadboard
 
Glass of water
 
Table salt
 
The resistance function is usually
shown by the Greek letter omega (Ω)
or sometimes by the word ohms
Set your multimeter to the highest
resistance range available
Touch the two test probes of your
meter together
When you do, the meter should
register 0 Ω of resistance
 
 
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Test the continuity of a piece of wire
by touching the probes to the bare
opposite ends of the wire. What
happens?
What does it show if the meter
doesn’t move at all when the probes
are touched to the wire?
 
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(Note: Some models say “OL” (open-loop), while
others display dashed lines.)
 
Use your meter to determine the continuity between the
holes on a breadboard. Which holes are connected?
 
Continuity
 
No Continuity
 
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Select a 33,000 Ω (33 kΩ) resistor.
For a 33 kΩ resistor, the three
coloured bands will be orange,
orange, orange.
The fourth coloured band
represents the tolerance of the
resistor. This might be gold (+/-
5%) or silver (+/- 10%).
 
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Set a suitable range on the ohmmeter scale
(e.g. 100 kΩ)
Touch the meter’s test probes to each end
of the 33 k
Ω 
resistor.
Be careful not to touch both probe tips to
your fingers at the same time – this will give
a measurement lower than it should be.
You should see a value close to 33 shown
on the display. Why is the number not
exactly 33?
 
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Try swapping which ends of the resistor the
test probes are touching.
Does this change the measured value?
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 resistance and
how it is measured?
 
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You can 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.
You may need to adjust the
meter range as your body
resistance may be greater than
50,000 ohms hand-to-hand.
 
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Try wetting the tips of your fingers
with tap water and re-measuring
resistance with the meter.
How does this affect the measured
value?
Repeat the process, this time wetting
your fingers with saltwater.
Does this affect the measured value?
If so, why?
 
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Take a piece of paper and draw a
very heavy black mark on it with a
pencil (not a pen!)
Measure the resistance of the black
strip with your meter, placing the
probe tips at each end of the mark
Move the probe tips closer together
on the black mark. How does the
resistance value change?
What does this teach you about
resistance versus the length of a
conductive material?
 
Mark made with pencil
 
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Investigate the resistance of other materials or substances.
For example:
Plastics
Woods
Paper
Metals
Glass
Fabrics
Clean water
Dirty water
 
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Explore the world of electrical resistance testing by learning how to measure resistivity, using a multimeter, and understanding the basics of resistance. Stay safe by following proper procedures and equipment guidelines throughout the testing process.


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  1. Explore electrical resistance Testing the electrical resistivity of different materials

  2. Stay safe Whether you are a scientist researching a new medicine or an engineer solving climate change, safety always comes first. An adult must always be around and supervising when doing this activity. You are responsible for: ensuring that any equipment used for this activity is in good working condition behaving sensibly and following any safety instructions so as not to hurt or injure yourself or others Please note that in the absence of any negligence or other breach of duty by us, this activity is carried out at your own risk. It is important to take extra care at the stages marked with this symbol:

  3. The Basics of Resistance Imagine water flowing through two pipes If one pipe has a narrow section this means that less water can flow through it. Less Resistance More Resistance The narrow pipe "resists" the flow of water.

  4. Measuring resistance Resistance to the flow of electricity can be measured using an ohmmeter This is one of the functions of a multimeter This is an electrical instrument capable of measuring voltage, current, and resistance Test probes

  5. Equipment needed Mulitimeter digital/analogue Assorted resistors Breadboard Crocodile clips Paper/pencil Glass of water Table salt

  6. Measuring resistance with a multimeter The resistance function is usually shown by the Greek letter omega ( ) or sometimes by the word ohms Set your multimeter to the highest resistance range available Touch the two test probes of your meter together When you do, the meter should register 0 of resistance

  7. WARNING: Do not attempt to measure the resistance of a battery using a multimeter set to the resistance ( ohms ) function. This could result in damaging the multimeter

  8. Continuity testing Test the continuity of a piece of wire by touching the probes to the bare opposite ends of the wire. What happens? What does it show if the meter doesn t move at all when the probes are touched to the wire? (Note: Some models say OL (open-loop), while others display dashed lines.)

  9. Continuity testing a breadboard Use your meter to determine the continuity between the holes on a breadboard. Which holes are connected? Continuity No Continuity

  10. Resistors Select a 33,000 (33 k ) resistor. For a 33 k resistor, the three coloured bands will be orange, orange, orange. The fourth coloured band represents the tolerance of the resistor. This might be gold (+/- 5%) or silver (+/- 10%).

  11. Measuring the resistance of a resistor Set a suitable range on the ohmmeter scale (e.g. 100 k ) Touch the meter s test probes to each end of the 33 k resistor. Be careful not to touch both probe tips to your fingers at the same time this will give a measurement lower than it should be. You should see a value close to 33 shown on the display. Why is the number not exactly 33?

  12. Investigating resistance Try swapping which ends of the resistor the test probes are touching. Does this change the measured value? 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 resistance and how it is measured?

  13. Measuring the resistance of a person You can 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. You may need to adjust the meter range as your body resistance may be greater than 50,000 ohms hand-to-hand.

  14. Factors that affect resistance Try wetting the tips of your fingers with tap water and re-measuring resistance with the meter. How does this affect the measured value? Repeat the process, this time wetting your fingers with saltwater. Does this affect the measured value? If so, why?

  15. Measuring the resistance of pencil lines Take a piece of paper and draw a very heavy black mark on it with a pencil (not a pen!) Measure the resistance of the black strip with your meter, placing the probe tips at each end of the mark Move the probe tips closer together on the black mark. How does the resistance value change? What does this teach you about resistance versus the length of a conductive material? Mark made with pencil

  16. Extension Investigate the resistance of other materials or substances. For example: Plastics Woods Paper Metals Glass Fabrics Clean water Dirty water

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