8.10 Res is tance, Capac i tance and Induc tance Measuremen t
In this section, we will examine and review the various means of measuring resistance,
capacitance and inductance. We have already examined null methods of measuring R, C
and L at audio frequencies using various bridge circuits in Chapters 4 and 5. These
methods will be referred to, but not repeated here. Rather, we will examine various active
and passive ohmmeter circuits, means of characterizing linear and nonlinear (voltage
variable) capacitances with dc and high frequency ac, and the means of measuring the
properties of inductances at high frequencies.
8.10.1 Resistance Measurements
Techniques have been developed to measure resistances from 107 to over 1014 .
Needless to say, specialized instruments must be used at the extreme ends of this range.
We have already seen in Chapter 4 that very accurate resistance measurements are
commonly made using dc Wheatstone or Kelvin bridges and a dc null detector such as
an electronic nanovoltmeter. The values of the resistances used in the arms of these
bridges must, of course, be known very accurately. We shall discuss other dc means of
measuring resistance below.
The voltmeter-ammeter method is probably the most basic means of measuring
resistance. It makes use of Ohm’s law and the assumption that the resistance is linear.
As shown in Figure 8.62, there are two basic configurations for this means of
measurement—the ammeter being before RX, which is in parallel with the voltmeter,
and the ammeter being in series with RX after the voltmeter. In the first case, the ammeter
measures the current in the voltmeter as well as RX, while in the second case, the
voltmeter measures the voltage drop across the ammeter plus that across RX. It is easy to
show that in the first case, RX is given by:
RX ¼ VX
I VX=RVM
ð8:138Þ
where VX is the voltmeter reading, I is the ammeter reading and RVM is the resistance
of the voltmeter (ideally, infinite). In general, RVM ¼VFS, where ¼ /V sensitivity of
the voltmeter and VFS is its full scale voltage range.
In the second case, we find:
RX ¼ V=IX RAM ð8:139Þ
8.10 Res is tance, Capac i tance and Induc tance Measuremen t
In this section, we will examine and review the various means of measuring resistance,
capacitance and inductance. We have already examined null methods of measuring R, C
and L at audio frequencies using various bridge circuits in Chapters 4 and 5. These
methods will be referred to, but not repeated here. Rather, we will examine various active
and passive ohmmeter circuits, means of characterizing linear and nonlinear (voltage
variable) capacitances with dc and high frequency ac, and the means of measuring the
properties of inductances at high frequencies.
8.10.1 Resistance Measurements
Techniques have been developed to measure resistances from 107 to over 1014 .
Needless to say, specialized instruments must be used at the extreme ends of this range.
We have already seen in Chapter 4 that very accurate resistance measurements are
commonly made using dc Wheatstone or Kelvin bridges and a dc null detector such as
an electronic nanovoltmeter. The values of the resistances used in the arms of these
bridges must, of course, be known very accurately. We shall discuss other dc means of
measuring resistance below.
The voltmeter-ammeter method is probably the most basic means of measuring
resistance. It makes use of Ohm’s law and the assumption that the resistance is linear.
As shown in Figure 8.62, there are two basic configurations for this means of
measurement—the ammeter being before RX, which is in parallel with the voltmeter,
and the ammeter being in series with RX after the voltmeter. In the first case, the ammeter
measures the current in the voltmeter as well as RX, while in the second case, the
voltmeter measures the voltage drop across the ammeter plus that across RX. It is easy to
show that in the first case, RX is given by:
RX ¼ VX
I VX=RVM
ð8:138Þ
where VX is the voltmeter reading, I is the ammeter reading and RVM is the resistance
of the voltmeter (ideally, infinite). In general, RVM ¼VFS, where ¼ /V sensitivity of
the voltmeter and VFS is its full scale voltage range.
In the second case, we find:
RX ¼ V=IX RAM ð8:139Þ
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