Chapter 21 Circuits and DC Instruments
21.4 DC Voltmeters and Ammeters
Summary
- Explain why a voltmeter must be connected in parallel with the circuit.
- Draw a diagram showing an ammeter correctly connected in a circuit.
- Describe how a galvanometer can be used as either a voltmeter or an ammeter.
- Find the resistance that must be placed in series with a galvanometer to allow it to be used as a voltmeter with a given reading.
- Explain why measuring the voltage or current in a circuit can never be exact.
Voltmeters measure voltage, whereas ammeters measure current. Some of the meters in automobile dashboards, digital cameras, cell phones, and tuner-amplifiers are voltmeters or ammeters. (See Figure 1.) The internal construction of the simplest of these meters and how they are connected to the system they monitor give further insight into applications of series and parallel connections.
Voltmeters are connected in parallel with whatever device’s voltage is to be measured. A parallel connection is used because objects in parallel experience the same potential difference. (See Figure 2, where the voltmeter is represented by the symbol V.)
Ammeters are connected in series with whatever device’s current is to be measured. A series connection is used because objects in series have the same current passing through them. (See Figure 3, where the ammeter is represented by the symbol A.)
Analog Meters: Galvanometers
Analog meters have a needle that swivels to point at numbers on a scale, as opposed to digital meters, which have numerical readouts similar to a hand-held calculator. The heart of most analog meters is a device called a galvanometer, denoted by G. Current flow through a galvanometer, [latex]{I_{\text{G}}}[/latex], produces a proportional needle deflection. (This deflection is due to the force of a magnetic field upon a current-carrying wire.)
The two crucial characteristics of a given galvanometer are its resistance and current sensitivity. Current sensitivity is the current that gives a full-scale deflection of the galvanometer’s needle, the maximum current that the instrument can measure. For example, a galvanometer with a current sensitivity of [latex]{50 \;\mu \text{A}}[/latex] has a maximum deflection of its needle when [latex]{50 \;\mu \text{A}}[/latex] flows through it, reads half-scale when [latex]{25 \;\mu \text{A}}[/latex] flows through it, and so on.
If such a galvanometer has a [latex]{25 - \;\Omega}[/latex] resistance, then a voltage of only [latex]{V = IR = (50 \;\mu \text{A}) (25 \;\Omega) = 1.25 \;\text{mV}}[/latex] produces a full-scale reading. By connecting resistors to this galvanometer in different ways, you can use it as either a voltmeter or ammeter that can measure a broad range of voltages or currents.
Galvanometer as Voltmeter
Figure 4 shows how a galvanometer can be used as a voltmeter by connecting it in series with a large resistance, [latex]{R}[/latex]. The value of the resistance [latex]{R}[/latex] is determi