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Instruments to Measure voltage, current and resistance

As we have seen in previous sections, an electric circuit is made up of a number of different components such as batteries, resistors and light bulbs. There are devices to measure the properties of these components. These devices are called meters.

For example, one may be interested in measuring the amount of current flowing through a circuit using an ammeter or measuring the voltage provided by a battery using a voltmeter. In this section we will discuss the practical usage of voltmeters, ammeters, and ohmmeters.

Voltmeter

A voltmeter is an instrument for measuring the voltage between two points in an electric circuit. In analogy with a water circuit, a voltmeter is like a meter designed to measure pressure difference. Since one is interested in measuring the voltage between two points in a circuit, a voltmeter must be connected in parallel with the portion of the circuit on which the measurement is made.

Figure 1: A voltmeter should be connected in parallel in a circuit.
Figure 1 (PG10C9_034.png)

Figure 1 shows a voltmeter connected in parallel with a battery. One lead of the voltmeter is connected to one end of the battery and the other lead is connected to the opposite end. The voltmeter may also be used to measure the voltage across a resistor or any other component of a circuit that has a voltage drop.

Ammeter

An ammeter is an instrument used to measure the flow of electric current in a circuit. Since one is interested in measuring the current flowing through a circuit component, the ammeter must be connected in series with the measured circuit component (Figure 2).

Figure 2: An ammeter should be connected in series in a circuit.
Figure 2 (PG10C9_035.png)

Ohmmeter

An ohmmeter is an instrument for measuring electrical resistance. The basic ohmmeter can function much like an ammeter. The ohmmeter works by suppling a constant voltage to the resistor and measuring the current flowing through it. The measured current is then converted into a corresponding resistance reading through Ohm's Law. Ohmmeters only function correctly when measuring resistance over a component that is not being powered by a voltage or current source. In other words, you cannot measure the resistance of a component that is already connected to a live circuit. This is because the ohmmeter's accurate indication depends only on its own source of voltage. The presence of any other voltage across the measured circuit component interferes with the ohmmeter's operation. Figure 3 shows an ohmmeter connected with a resistor.

Figure 3: An ohmmeter should be used when there are no voltages present in the circuit.
Figure 3 (PG10C9_036.png)

Meters Impact on Circuit

A good quality meter used correctly will not significantly change the values it is used to measure. This means that an ammeter has very low resistance to not slow down the flow of charge. A voltmeter has a very high resistance so that it does not add another parallel pathway to the circuit for the charge to flow along.

Investigation : Using meters

If possible, connect meters in circuits to get used to the use of meters to measure electrical quantities. If the meters have more than one scale, always connect to the largest scale first so that the meter will not be damaged by having to measure values that exceed its limits.

The table below summarises the use of each measuring instrument that we discussed and the way it should be connected to a circuit component.

Table 1
Instrument Measured Quantity Proper Connection
Voltmeter Voltage In Parallel
Ammeter Current In Series
Ohmmeter Resistance Only with Resistor
Figure 4
Khan academy video on circuits - 4

The following presentation summarizes the concepts covered in this chapter.

Figure 5

Exercises - Electric circuits

  1. Write definitions for each of the following:
    1. resistor
    2. coulomb
    3. voltmeter
    Click here for the solution
  2. Draw a circuit diagram which consists of the following components:
    1. 2 batteries in parallel
    2. an open switch
    3. 2 resistors in parallel
    4. an ammeter measuring total current
    5. a voltmeter measuring potential difference across one of the parallel resistors
    Click here for the solution
  3. Complete the table below:
    Table 2
    QuantitySymbolUnit of meaurementSymbol of unit
    e.g. Distancee.g. de.g. kilometere.g. km
    Resistance   
    Current   
    Potential difference   
    Click here for the solution
  4. [SC 2003/11] The emf of a battery can best be explained as the ⋯⋯
    1. rate of energy delivered per unit current
    2. rate at which charge is delivered
    3. rate at which energy is delivered
    4. charge per unit of energy delivered by the battery
    Click here for the solution
  5. [IEB 2002/11 HG1] Which of the following is the correct definition of the emf of a battery?
    1. It is the product of current and the external resistance of the circuit.
    2. It is a measure of the cell's ability to conduct an electric current.
    3. It is equal to the “lost volts” in the internal resistance of the circuit.
    4. It is the power supplied by the battery per unit current passing through the battery.
    Click here for the solution
  6. [IEB 2005/11 HG] Three identical light bulbs A, B and C are connected in an electric circuit as shown in the diagram below.
    Figure 6
    Figure 6 (PG10C9_037.png)
    1. How bright is bulb A compared to B and C?
    2. How bright are the bulbs after switch S has been opened?
    3. How do the currents in bulbs A and B change when switch S is opened?
      Table 3
       Current in ACurrent in B
      (a)decreasesincreases
      (b)decreasesdecreases
      (c)increasesincreases
      (d)increasesdecreases
    Click here for the solution
  7. [IEB 2004/11 HG1] When a current II is maintained in a conductor for a time of tt, how many electrons with charge e pass any cross-section of the conductor per second?
    1. It
    2. It/e
    3. Ite
    4. e/It
    Click here for the solution

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