A voltmeter is a device for measuring voltage. It measures the voltage
drop from the red to the black probes. The voltmeter is placed
in parallel with the circuit element whose voltage is to be measured.
Recall that two elements are in parallel when they share the same
pair of nodes and hence share the same voltage.
Consider the voltage divider circuit shown in Figure 
in which
the voltage across 
is to be measured. If the presence of the voltmeter
does not affect the voltage it is intending to measure, the meter must
draw no current. That is, it must act as an open circuit. An open
circuit may be thought of as an infinite resistance. Hence, an ideal
voltmeter has an infinite resistance. You measured the internal resistance
of the voltmeter in Experiment 2 and found the value to be on the order of
M
which is large, but certainly not infinite.
Figure: Voltage Divider Circuit
First consider the circuit with the voltmeter not present. In this case the voltage
can be expressed in terms of the source voltage 
and the resistors
and 
by
With the voltmeter present, its resistance alters the voltage division equation which becomes
where 
is the resistance of the voltmeter. You will not be able to see
how this equation was obtained at first examination. Let the voltmeter in
Figure be represented by a resistance 
. Use resistance
reduction and voltage division to obtain an expression for 
in terms
of 
. Then, clear the fractions in the numerator and denominator. Be
sure to show your derivation in your lab report. Recall that an ideal
voltmeter has infinite resistance. Letting the value of 
in Equation
be infinite should result in Equation . Derive equation
from Equation by taking the limit as 
.
L'Hospital's Rule may be helpful.
You will now build the voltage divider circuit using the DC power supply as
the voltage source 
in Figure .
