Understand input resistance of voltmeters and ammeters
Investigate input resistance as a source of measurement error
Preparation:
Review Ohm's Law, the voltage divider law, and the current divider law.
Equipment and Parts:
Power Supply
Digital Multimeter
Analog Ammeter with 1 mA scale
Resistors: 5.1 MΩ, 10 kΩ, & 100 Ω
Procedure:
Determine the input resistance of the voltmeter using a voltage divider.
Set power supply output voltage to 8 V. Measure and adjust the output voltage using
the voltmeter.
Connect the voltmeter in series with a 5.1 MΩ resistor and with the power supply.
(Draw a picture of this circuit in your lab book.)
The voltmeter is measuring it's own voltage due to its input resistance. Calculate
the the voltmeter's input resistance using the voltage divider law.
Replace the 5.1 MΩ resistor with a resistor as close to the value of the voltmeter
input resistance as possible. The voltmeter's voltage should now be about 4 V.
Recalculate the voltmeter's input resistance using the voltage divider law.
Determine the input resistance of the ammeter using a current divider.
Place a 10 kΩ current-limiting resistor in series with the output of the power supply.
Connect the ammeter in parallel with a 100 Ω resistor. Connect this combination in
series with the power supply and current-limiting resistor. (Draw a picture of this
circuit in your lab book.)
Adjust the power supply output voltage so that the voltage across the current-limiting
resistor is 10 V. The power supply is now supplying a total current of 1 mA.
The ammeter is measuring it's own current based on its input resistance. Calculate
the ammeter's input resistance using the current divider law.
Replace the 100 Ω resistor with a resistor as close to the calculated input resistance
of the ammeter. The ammeter's current should now be about 0.5 mA.
Recalculate the ammeter's input resistance using the current divider law.
Determine the input resistance of the ammeter using Ohm's Law.
Use the same 10 kΩ current-limiting resistor as before.
Connect the ammeter in parallel with the voltmeter. Connect this combination in series
with the power supply and current-limiting resistor.
Slowly turn up the supply voltage until the ammeter has gone full scale.
The ammeter is measuring its own current. The voltmeter is measuring the voltage across
the ammeter. Calculate the ammeter's input resistance using ohm's law.
Conclusions:
Compare two measurements of the ammeters' input resistance.
Describe a scenario where the voltmeter would be a significant source of measurement
error.
Describe a scenario when the ammeter would be a significant source of measurement
error.
