17.5. Measuring Current and Voltage http://www.ck12.org
17.5 Measuring Current and Voltage
Objectives
The student will:
- Understand how an ammeter is used.
- Understand how a voltmeter is used.
Vocabulary
- ammeter:A device used to measure currents.
- voltmeter:A device used to measure voltages.
Introduction
There are two basic devices used to measure the currents and voltages of electrical circuits. Theammeteris used to
measure currents and avoltmeteris used to measure voltages.
The Ammeter
An ammeter is always inserted in series with other electronic components, since it is used to measure the current in
a circuit.
Figure17.15 shows two ammetersA 1 and A 2 reading the current through two different parts of the circuit.
AmmeterA 1 reads the battery current through the circuit. SinceFigure17.15 is identical to the diagram of Example
18.3.2, the ammeter reads a current ofA 1 =^109 A..
AmmeterA 2 reads the current through the 12Ωresistor:A 2 =^2027 A.
Ammeters must have a very small resistance. Otherwise, they would add a significant amount of resistance to the
circuit and affect the very current they are meant to measure. It is very important not to connect an ammeter in
parallel in a circuit. As stated above, a small resistance in such circumstances draws a large current. The ammeter
may well be damaged from the resulting joule heating.
The Voltmeter
A voltmeter is always connected in parallel with other electronic components since it is used to measure the potential
difference across a circuit element.