All meters have
resistance.
The value of this
resistance depends upon
the voltage range
selected.
A typical moving coil
meter has a SENSITIVITY
of 20,000 ohms per volt.
This means that when the
1 volt range is selected
the meter has a
resistance of 20,000
ohms.
When the 10 volt range
is selected it has a
resistance of 200,000
ohms and so on.
When the meter is
connected to a circuit
to measure voltage, this
resistance will affect
the circuit and
therefore the accuracy
of the measurement
obtained.
In Fig.1 the voltage
across each resistor can
be calculated.
However, it can be shown
that since the resistors
are of the same value
then the battery voltage
divides equally across
them, and the voltage
across each will be 15
volts.
Now if we set the meter
to the 20 volt range to
measure this voltage,
its resistance will be
20 x 20,000 = 400,000
ohms = 400k.
If we connect it across
the top resistor, as in
Fig.2 then we have two
400k resistors in
parallel.
Calculating the result
of this gives us 200,000
ohms and the circuit
looks like Fig.3
The voltage will now
divide to give 10 volts
across the top resistor
and 20 volts across the
lower resistor.
The meter will indicate
10 volts when we know
that it should indicate
15 volts.
Similarly, connecting
the meter across the
lower resistor will
again indicate 10 volts.
It appears that there is
10v + 10v = 20 volts
across the two
resistors, when in fact
there is 30 volts.
To obtain the most
accurate results, set
the meter on the highest
range possible.
This means that its
resistance will be
highest and have least
effect on the circuit.
Digital meter have a
very high resistance,
typically 10 Megohms on
all ranges, and the
readings obtained are
more accurate than those
obtained using a moving
coil meter.
When buying a new meter
look for a sensitivity
greater than 20,000
ohms/volt.
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