Objective
Learn typical scale of resistance units.
Metals are generally good at conducting electricity; they have low
resistance. Common insulators (including air) have a resistance high enough
it can't be easily measured. So the most basic resistance measurement is
the 'continuity check' which simply tests whether the resistance is low
enough to be considered a conductor, or whether it is unexpectedly high
representing an open circuit.
Steps and observations
- Set the DMM to resistance: Ohms, or the large greek omega letter
Ω. The DMM will emit a small voltage to measure the current which
flows in response through the resistance path between the probes.
- Hold the probe tips in your fingers. This is safe, the DMM voltage
emitted is very low. Squeeze harder. Moisten the contact point. You
should see a body resistance reading which changes with conditions.
- Touch the probes to a coin, keys, and metal tool surfaces. If you
fail to see a low reading, try changing contact points, scratching the
surface, and pressing harder.
- Touch the probes across the leads of a resistor. The quarter-watt
resistors with wire leads we use in the lab
have colored
bands which encode the specified value.
- Touch the probes across the leads of an LED. Reverse the
connections.
- Try measuring resistance through other natural objects, food,
furniture, etc. Write down your results.
Comments
Typical resistance measurements along a short wire will be less than 1
ohm and are often dominated by the resistance of the contact point and probe
wires. Component resistors are available in standard ranges from a fraction
of an ohm to tens of millions of ohms. For our typical sensor circuits we
will be using resistors in the 100 to 10000 (10K) ohm range.