What Is a Voltmeter and an Ammeter?

A voltmeter measures electric potential difference (voltage) between two points, while an ammeter measures the flow of electric charge (current) through a circuit. In practice, a voltmeter is connected in parallel across a component and has very high internal resistance; an ammeter is connected in series with the circuit and has very low internal resistance to minimize its effect on the measurement.

Fundamentals: Definitions and How They Work

A voltmeter is designed to quantify how much “push” charges experience between two nodes, expressed in volts (V). Because it sits in parallel with the circuit section under test, it must draw negligible current; modern digital voltmeters typically present 10 MΩ or higher input impedance (some add a selectable low-impedance “LoZ” mode to bleed ghost voltages on long cables).

An ammeter measures the rate of charge flow, expressed in amperes (A). Inline (series) ammeters insert a very low-value shunt resistor and infer current from the tiny voltage drop across it; the lower this “burden voltage,” the less the meter disturbs the circuit. Non-contact clamp meters instead sense current via a current transformer (AC) or Hall-effect sensor (AC/DC), avoiding a series break.

How to Connect and Use Them Safely

The following steps outline safe, general procedures for using a voltmeter and an ammeter with a digital multimeter or dedicated instruments. Always consult your instrument’s manual and follow applicable safety standards.

1. De-energize the circuit if possible. Inspect leads for damage and confirm the meter’s category rating suits the environment (IEC 61010 CAT II/III/IV).
2. For voltage: select the appropriate voltage function (AC or DC) and start at a higher range if the level is unknown.
3. Connect the voltmeter in parallel: black lead to the lower potential (common/ground), red lead to the higher potential or the node of interest; then energize and read.
4. For current: move the red lead to the correct fused current jack (mA or A), select AC or DC current, and start on the highest current range.
5. Break the circuit and insert the ammeter in series so all current flows through the meter; energize, read, then de-energize before removing the meter.

These steps reduce risk of meter damage, blown fuses, and unintended circuit disturbances. Parallel for voltage and series for current is the rule that prevents short circuits and protects both you and the equipment.

Types and Features

Voltmeter Types

Different voltmeters are optimized for accuracy, waveform type, and application. The list below highlights common options and where they fit.

• Digital voltmeters (DMMs/DVMs): general-purpose, 3.5 to 6.5 digits; high input impedance; many offer true-RMS measurement for accurate AC readings on non-sinusoidal waveforms.
• Analog moving-coil (DC) and moving-iron (AC) meters: useful for observing trends and needle deflection; lower precision; sensitive to orientation and parallax.
• Data-logging voltmeters: record trends over time; helpful for battery testing, solar PV monitoring, and troubleshooting intermittent faults.
• Specialized high-impedance probes: for sensitive circuits or oscilloscopes; include attenuators and compensation networks.

For most electronics and electrical work, a true-RMS digital multimeter with at least 10 MΩ input impedance and adequate resolution (e.g., 60,000 counts) is the practical default.

Ammeter Types

Ammeter selection depends on whether you can break the circuit, the current level, and whether it is AC, DC, or pulsed. The following options address typical scenarios.

• Inline shunt ammeters (DMM current ranges): measure by dropping a small voltage across an internal resistor; precise but introduce “burden voltage.”
• External shunt + voltmeter method: for higher currents; measure millivolts across a known shunt (e.g., 50 mV at full scale) to infer current.
• Clamp meters (CT for AC, Hall-effect for AC/DC): non-contact measurement without opening the circuit; excellent for mains troubleshooting and automotive DC.
• USB/bench supply inline meters: purpose-built for low-voltage electronics, providing current, voltage, and sometimes energy readings.

If you cannot interrupt a live circuit, a clamp meter—especially a Hall-effect model for DC—is the safest choice and avoids altering circuit behavior.

Accuracy, Range, and What the Numbers Mean

Manufacturers specify performance in ways that affect how you interpret readings. The points below clarify the most important terms and trade-offs.

• Units: voltage in volts (V); current in amperes (A). Milliamp (mA) and microamp (µA) ranges are common for low-current electronics.
• Resolution vs accuracy: resolution is the smallest increment displayed; accuracy is how close the reading is to the true value (often stated as percent of reading plus counts).
• Burden voltage (ammeter): the extra voltage drop introduced during current measurement; low-burden meters reduce circuit disturbance and heat.
• Input impedance (voltmeter): higher is better for not loading the circuit; typical DMM is 10–11 MΩ, while LoZ (~1 kΩ) is used to eliminate ghost voltages on long cables.
• Frequency response and true RMS: for AC with distortion or PWM, use a true-RMS meter with sufficient bandwidth and crest-factor capability.
• Range protection: fused current inputs (often HRC ceramic fuses) and overvoltage protection protect users and instruments.

Understanding these specs helps you select the right instrument and correctly judge uncertainty, especially when measuring small signals or complex waveforms.

Safety and Best Practices

Voltage and current measurement can expose users to hazardous energy. The following practices significantly reduce risk and improve measurement reliability.

• Check IEC 61010 CAT rating (II/III/IV) and maximum voltage; match the meter and leads to the environment (e.g., CAT III for distribution panels).
• Use the correct jack: move the red lead to the fused A or mA input only when measuring current; return it to the V/Ω jack for voltage and resistance.
• Never place an ammeter directly across a voltage source—it will act like a short and can blow fuses or cause damage.
• De-energize before inserting an inline ammeter; verify absence of voltage where feasible and follow lockout/tagout in industrial settings.
• Keep one hand behind your back on high-voltage work to reduce shock path risk; wear appropriate PPE.
• Replace blown fuses only with the specified type and rating; inspect leads regularly and use finger guards.

Adhering to these precautions aligns with industry standards and helps prevent injuries and equipment failures during measurement.

Common Mistakes and How to Avoid Them

Misuse often stems from simple setup errors. The list below highlights frequent pitfalls and quick fixes.

• Leads left in the A/mA jack when switching back to voltage: the meter may short the source when probes touch; always verify lead placement.
• Wrong range or function (AC vs DC): confirm settings before contact; start high and step down.
• Blown internal current fuse: if current always reads zero, check and replace the fuse with the specified HRC type.
• Ignoring burden voltage: sensitive circuits may misbehave when measured in series; use a low-burden meter or external shunt.
• Clamp meter zeroing: for DC, zero the sensor before measurement and center the conductor in the jaw to reduce error.
• Measuring across unknown high-energy sources with a low-CAT meter: upgrade meter and leads to suitable CAT rating.

A brief pre-check—function, range, jacks, and condition—avoids most measurement setbacks and ensures dependable results.

When to Use a Multimeter or a Clamp Meter

Choosing the right tool depends on current level, the need to break a circuit, and the type of signal. The guidance below can help you decide.

• Digital multimeter: best for general voltage and low-to-moderate current, resistance, and continuity checks with high accuracy.
• Clamp meter: best for moderate to high currents, especially on live mains or automotive DC, without opening the circuit.
• External shunt + voltmeter: best for high currents or permanent monitoring with minimal burden.
• USB/power analyzers: best for low-voltage electronics and power profiling (e.g., phone chargers, IoT devices).
• Oscilloscope with current/voltage probes: best for transients, ripple, PWM, and detailed waveform analysis.

Matching the instrument to the job improves safety and accuracy, and often speeds up troubleshooting.

Key Differences at a Glance

The points below summarize how voltmeters and ammeters differ in purpose and behavior within a circuit.

• Quantity measured: voltmeter—electric potential difference (V); ammeter—current (A).
• Connection: voltmeter—parallel; ammeter—series.
• Internal resistance: voltmeter—very high; ammeter—very low (plus burden voltage consideration).
• Effect on circuit: voltmeter—minimizes loading; ammeter—introduces small voltage drop.
• Symbols: V for volts; A for amperes; common schematic symbols use circle with V or A.
• Typical tools: DMM for both; clamp meter for current without breaking the circuit.

Together, these distinctions guide correct instrument choice and connection to obtain accurate, safe measurements.

Summary

A voltmeter measures voltage across two points and is connected in parallel with high input impedance; an ammeter measures current through a circuit and is connected in series with low resistance or via a non-contact clamp. Selecting the right type, understanding specs like true RMS, burden voltage, and input impedance, and following IEC 61010 safety practices ensure accurate, safe, and reliable electrical measurements.

What is a voltmeter and an ammeter?

Ammeters measure electric current in amps, while voltmeters measure voltage. In general, you’ll use an ammeter to measure currents in a circuit, and you’ll use a voltmeter to measure potential difference or electromotive force (voltage).

Is a multimeter a voltmeter and ammeter?

Ammeter measures current in a wire to be connected in series in the circuit. Voltmeter measures potential difference between 2 points, to be connected in parallel to those 2 points. Multimeter is a device that can include an ammeter, voltmeter and ohmmeter.

What does an ammeter do?

An ammeter measures and displays the electric current (or flow of electricity) in an electric circuit, using a low-resistance device connected in series with the circuit component being measured. To get a reading, the current must pass directly through the ammeter. Ammeters are available in both analog (mechanical needle on a scale) and digital (numerical display) forms, and are a common function within a multimeter.
 
How it works

1. Series Connection: Opens in new tabUnlike a voltmeter, which is connected in parallel, an ammeter must be connected in series with the circuit element whose current you want to measure. This means the circuit must be temporarily broken to insert the ammeter into the path of the current. 
2. Low Resistance: Opens in new tabAmmeters are designed with very low internal resistance. This is crucial so that the ammeter doesn’t significantly alter the amount of current flowing in the circuit, which would lead to an inaccurate measurement. 
3. Measuring the Current: Opens in new tabAs the electric current flows through the circuit and through the ammeter, the device displays the magnitude of the current, typically in amperes (amps), milliamperes, or microamperes. 

Types of Ammeters

• Analog Ammeters: These use a moving coil or a moving iron that deflects a needle across a graduated scale. 
• Digital Ammeters: These display the current as a numerical value on a digital screen. 
• Clamp-on Ammeters: A variation that can measure current without breaking the circuit by using a magnetic field generated by the current. 

Key characteristics 

• Purpose: Measures electric current.
• Connection: Always connected in series with the component being measured.
• Resistance: Has very low internal resistance.
• Unit of measurement: Amperes (amps).

What is a voltmeter in simple terms?

A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit.