What is the electrical equipment used to know if there is an electric current flow in a source?

Current is the rate at which electrons flow past a point in a complete electrical circuit. At its most basic, current = flow.

An ampere (AM-pir), or amp, is the international unit used for measuring current. It expresses the quantity of electrons (sometimes called "electrical charge") flowing past a point in a circuit over a given time.

A current of 1 ampere means that 1 coulomb of electrons—that's 6.24 billion billion (6.24 x 1018) electrons—is moving past a single point in a circuit in 1 second. The calculation is similar to measuring water flow: how many gallons pass a single point in a pipe in 1 minute (gallons per minute, or GPM).

Symbols used for amps:

A = amperes, for a large amount of current (1.000).
mA = milliamperes, a thousandth of an amp (0.001).
µA = microamperes, a millionth of an amp (0.000001).

In formulas such as Ohm's Law, current is also represented by I (for intensity).

Amps are named for French mathematician/physicist Andrè-Marie Ampére (1775-1836), credited for proving:

• A magnetic field is generated around a conductor as current passes through it.
• The strength of that field is directly proportional to the amount of current flowing.

Electrons flow through a conductor (typically a metal wire, usually copper) when two prerequisites of an electric circuit are met:

1. The circuit includes an energy source (a battery, for instance) that produces voltage. Without voltage, electrons move randomly and fairly evenly within a wire, and current cannot flow. Voltage creates pressure that drives electrons in a single direction.
2. The circuit forms a closed, conducting loop through which electrons can flow, providing energy to any device (a load) connected to the circuit. A circuit is closed (complete) when a switch is turned to the ON, or closed, position (see diagram at the top of this page).

Current, like voltage, can be direct or alternating.

Direct current (dc):

• Represented by the symbols
  
  or
  
  on a digital multimeter.
• Flows only in one direction.
• Common source: batteries or dc generator.

Alternating current (ac):

• Represented by the symbols
  
  or
  
  on a digital multimeter.
• Flows in a sine wave pattern (shown below); reverses direction at regular intervals.
• Common source: household electrical receptacles powered by a public utility.

Most digital multimeters can measure dc or ac current no higher than 10 amps. Higher current must be scaled down with a current clamp accessory, which measures current (from .01 A or less to 1000 A) by gauging the strength of the magnetic field around a conductor. This permits measurements without opening the circuit.

Any component (lamp, motor, heating element) that converts electrical energy into some other form of energy (light, rotating motion, heat) uses current.

When additional loads are added to a circuit, the circuit must deliver more current. The size of conductors, fuses and the components themselves will determine how much current will flow through the circuit.

Amperage measurements are normally taken to indicate the amount of circuit loading or the condition of a load. Measuring current is a standard part of troubleshooting.

Current flows only when voltage provides the necessary pressure to cause electrons to move. Different voltage sources produce different amounts of current. Standard household batteries (AAA, AA, C and D) produce 1.5 volts each, yet larger batteries are capable of delivering greater amounts of current.

Recommended Resources:

Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.

Sharif Tarabay/Getty Images

What we call electrical current occurs on the particle level among the atoms of a conducting material—in a household circuit, this is the copper wiring. In each atom there are three types of particles: neutrons, protons (which carry a positive electromagnetic charge) and electrons (which carry a negative charge). The important particle here is the electron, since it has the unique characteristic of being able to separate from its atom and move to an adjacent atom. This flow of electrons is what creates electrical current—the jump of negatively-charged electrons from atom to atom.

What sends the electrons into motion? The physics are complicated, but in essence, electrical flow in circuit wires is made possible by a utility generator (a turbine powered by wind, water, an atomic reactor, or burning fossil fuels). In 1831, Michael Faraday discovered that electrical charges were created when a material that conducts electricity (metal wire) is moved within a magnetic field. This is the principal by which modern generators work: The turbines—whether powered by falling water or steam created by nuclear reactors—rotate huge coils of metal wire inside giant magnets, thereby causing electrical charges to flow.

With this massive electrical field of positive and negative charges established, the electrons in the wires throughout the power grid jump into action and begin to flow in cadence with the electrical field. When you flip a light switch or plug in a lamp or toaster, you are actually tapping into a large utility-wide flow of electrons being pulled and pushed by utility generators that may be hundreds of miles away.

Electrical generators are sometimes likened to water pumps—they do not create the electricity (just like a water pump does not create water), but they make the flow of electrons possible.

Voltmeters are tools used to measure the potential difference between two points in a circuit. The voltmeter is connected in parallel with the element to be measured, meaning an alternate current path around the element to be measured and through the voltmeter is created. You have connected a voltmeter correctly if you can remove the voltmeter from the circuit without breaking the circuit. In the diagram at right, a voltmeter is connected to correctly measure the potential difference across the lamp. Voltmeters have very high resistance so as to minimize the current flow through the voltmeter and the voltmeter's impact on the circuit.

Ammeters

Ammeters are tools used to measure the current in a circuit. The ammeter is connected in series with the circuit, so that the current to be measured flows directly through the ammeter. The circuit must be broken to correctly insert an ammeter. Ammeters have very low resistance to minimize the potential drop through the ammeter and the ammeter's impact on the circuit, so inserting an ammeter into a circuit in parallel can result in extremely high currents and may destroy the ammeter. In the diagram at right, an ammeter is connected correctly to measure the current flowing through the circuit.

Question: In the electric circuit diagram at right, possible locations of an ammeter and a voltmeter are indicated by circles 1, 2, 3, and 4. Where should an ammeter be located to correctly measure the total current and where should a voltmeter be located to correctly measure the total voltage?

Answer: To measure the total current, the ammeter must be placed at position 1, as all the current in the circuit must pass through this wire, and ammeters are always connected in series.

To measure the total voltage in the circuit, the voltmeter could be placed at either position 3 or position 4. Voltmeters are always placed in parallel with the circuit element being analyzed, and positions 3 and 4 are equivalent because they are connected with wires (and potential is always the same anywhere in an ideal wire).

Question: Which circuit diagram below correctly shows the connection of ammeter A and voltmeter V to measure the current through and potential difference across resistor R?

Answer: (4) shows an ammeter in series and a voltmeter in parallel with the resistor.

Question: Compared to the resistance of the circuit being measured, the internal resistance of a voltmeter is designed to be very high so that the meter will draw

1. no current from the circuit
2. little current from the circuit
3. most of the current from the circuit
4. all the current from the circuit

Answer: (2) the voltmeter should draw as little current as possible from the circuit to minimize its effect on the circuit, but it does require some small amount of current to operate.