A meter is a measuring
instrument. An ammeter measures current, a
voltmeter measures the potential difference
(voltage) between two points, and an ohmmeter
measures resistance. A multimeter combines
these functions, and possibly some additional ones as well,
into a single instrument.
Before going in to detail about
multimeters, it is important for you to have a clear idea of
how meters are connected into circuits. Diagrams A
and B below show a circuit before and after
connecting an ammeter:
A |

|
B |

|
to measure current, the
circuit must be broken to allow the
ammeter to be connected in series |
ammeters must have a LOW
resistance |
Think about the changes you
would have to make to a practical circuit in order to
include the ammeter. To start with, you need to break
the circuit so that the ammeter can be connected in
series. All the current flowing in the circuit must pass
through the ammeter. Meters are not supposed to alter the
behaviour of the circuit, or at least not significantly, and
it follows that an ammeter must have a very LOW resistance.
Diagram C shows the
same circuit after connecting a voltmeter:
A |

|
C |

|
to measure potential
difference (voltage), the circuit is not changed:
the voltmeter is connected in parallel |
voltmeters must have a
HIGH resistance |
This time, you do not need to
break the circuit. The voltmeter is connected in parallel
between the two points where the measurement is to be made.
Since the voltmeter provides a parallel pathway, it should
take as little current as possible. In other words, a
voltmeter should have a very HIGH resistance.
Which measurement technique
do you think will be the more useful? In fact, voltage
measurements are used much more often than current
measurements.
The processing of electronic
signals is usually thought of in voltage terms. It is an
added advantage that a voltage measurement is easier to
make. The original circuit does not need to be changed.
Often, the meter probes are connected simply by touching
them to the points of interest.
An ohmmeter does not function
with a circuit connected to a power supply. If you want to
measure the resistance of a particular component, you must
take it out of the circuit altogether and test it
separately, as shown in diagram D:
A |

|
D |

|
to measure resistance,
the component must be removed from the circuit
altogether |
ohmmeters work by
passing a current through the component being tested |
Ohmmeters work by passing a
small current through the component and measuring the
voltage produced. If you try this with the component
connected into a circuit with a power supply, the most
likely result is that the meter will be damaged. Most
multimeters have a fuse to help protect against misuse. |