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. (see the page on voltage
dividers).
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. |
