16:220 - Internal Resistance
1
Laboratory 6
INTERNAL RESISTANCE
Objective:
Measure the internal resistance of a battery and a voltmeter and determine when the
effect of the internal resistance is significant.
Introduction:
Ideal components (batteries, voltmeters, etc.) have no internal resistance; however, real
components have an internal resistance that affects its performance in some (but not all)
applications.
Apparatus:
Battery, resistors, 0-10 K variable resistor, microvoltmeter, digital voltmeter
(DVM)
A. Internal Resistance of a Voltmeter
Introduction:
An ideal voltmeter draws no current; however, when a real voltmeter makes a
measurement, it draws a current because it has an internal resistance in parallel with an ideal
voltmeter.
Procedure:
Measure V1, the potential of the battery. Place a 10 M! resistor in series with the
standard voltmeter. Measure V2, the new potential. Prove that
V2/V1 = r/(r + 10 M!)
Calculate r, the internal resistance of the voltmeter.