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  4. Direct-current circuits
  5. Electrical measuring instruments

10.4 Electrical measuring instruments

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By the end of the section, you will be able to:
  • Describe how to connect a voltmeter in a circuit to measure voltage
  • Describe how to connect an ammeter in a circuit to measure current
  • Describe the use of an ohmmeter

Ohm’s law and Kirchhoff’s method are useful to analyze and design electrical circuits, providing you with the voltages across, the current through, and the resistance of the components that compose the circuit. To measure these parameters require instruments, and these instruments are described in this section.

Dc voltmeters and ammeters

Whereas voltmeter     s measure voltage, ammeter     s measure current. Some of the meters in automobile dashboards, digital cameras, cell phones, and tuner-amplifiers are actually voltmeters or ammeters ( [link] ). 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.

The figure shows photo of fuel and temperature gauges.
The fuel and temperature gauges (far right and far left, respectively) in this 1996 Volkswagen are voltmeters that register the voltage output of “sender” units. These units are proportional to the amount of gasoline in the tank and to the engine temperature. (credit: Christian Giersing)

Measuring current with an ammeter

To measure the current through a device or component, the ammeter is placed in series with the device or component. A series connection is used because objects in series have the same current passing through them. (See [link] , where the ammeter is represented by the symbol A.)

Part a shows positive terminal of a battery with emf ε and internal resistance r connected to ammeter which is connected in series to two resistors, R subscript 1 and R subscript 2. Part b shows positive terminal of a battery with emf ε and internal resistance r connected to ammeter which is connected to two parallel resistors, R subscript 1 with ammeter and R subscript 2 with ammeter.
(a) When an ammeter is used to measure the current through two resistors connected in series to a battery, a single ammeter is placed in series with the two resistors because the current is the same through the two resistors in series. (b) When two resistors are connected in parallel with a battery, three meters, or three separate ammeter readings, are necessary to measure the current from the battery and through each resistor. The ammeter is connected in series with the component in question.

Ammeters need to have a very low resistance, a fraction of a milliohm. If the resistance is not negligible, placing the ammeter in the circuit would change the equivalent resistance of the circuit and modify the current that is being measured. Since the current in the circuit travels through the meter, ammeters normally contain a fuse to protect the meter from damage from currents which are too high.

Measuring voltage with a voltmeter

A voltmeter is connected in parallel with whatever device it is measuring. A parallel connection is used because objects in parallel experience the same potential difference. (See [link] , where the voltmeter is represented by the symbol V.)

Part a shows positive terminal of a battery with emf ε and internal resistance r connected in series to two resistors, R subscript 1 and R subscript 2. The battery and the two resistors have voltmeters connected to them in parallel.
To measure potential differences in this series circuit, the voltmeter (V) is placed in parallel with the voltage source or either of the resistors. Note that terminal voltage is measured between the positive terminal and the negative terminal of the battery or voltage source. It is not possible to connect a voltmeter directly across the emf without including the internal resistance r of the battery.

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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