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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 andlight 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 .


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

A voltmeter should be connected in parallel in a circuit.

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


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

An ammeter should be connected in series in a circuit.


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

An ohmmeter should be used when there are no voltages present in the circuit.

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 hasvery 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 tomeasure 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 itslimits.

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

Instrument Measured Quantity Proper Connection
Voltmeter Voltage In Parallel
Ammeter Current In Series
Ohmmeter Resistance Only with Resistor

Khan academy video on circuits - 4

The following presentation summarizes the concepts covered in this chapter.

Exercises - electric circuits

  1. Write definitions for each of the following:
    1. resistor
    2. coulomb
    3. voltmeter
  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
  3. Complete the table below:
    Quantity Symbol Unit of meaurement Symbol of unit
    e.g. Distance e.g. d e.g. kilometer e.g. km
    Potential difference
  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
  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.
  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.
    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?
      Current in A Current in B
      (a) decreases increases
      (b) decreases decreases
      (c) increases increases
      (d) increases decreases
  7. [IEB 2004/11 HG1] When a current I is maintained in a conductor for a time of t , 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

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
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What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
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Damian Reply
absolutely yes
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it is a goid question and i want to know the answer as well
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Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Siyavula textbooks: grade 10 physical science. OpenStax CNX. Aug 29, 2011 Download for free at http://cnx.org/content/col11245/1.3
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