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General instrument information for lab staff and core-facility users. It is not a purchasing recommendation and gives no prices.
Short answer
An electrometer is a very high-input-impedance DC meter for voltages, currents, resistances and charges that are too small, or sit behind too high a source resistance, for an ordinary multimeter. The usual modern electrometer combines four functions in one instrument: voltmeter, ammeter, ohmmeter and coulombmeter (Keithley, Low Level Measurements Handbook, 7th ed.). A picoammeter is a related, narrower instrument built along the lines of the electrometer’s ammeter function.
What is an electrometer? Definition and scope
Keithley’s handbook lists the electrometer among the instruments used for DC measurements, alongside digital multimeters (DMMs), nanovoltmeters, picoammeters and source measure units. What sets the electrometer apart is the combination of extremely high input resistance, very low input bias current and very low voltage burden. Those properties mean the instrument disturbs the circuit or sample it is measuring far less than a general-purpose meter would.
The term is used here for the modern solid-state, feedback-type instrument described by manufacturers’ low-level measurement literature. It is the instrument found in labs doing ion-beam, photodiode, mass-spectrometer, insulator and high-impedance electrode work.
How does an electrometer work?
An electrometer voltmeter draws almost nothing from the circuit under test. Keithley states its input resistance is typically greater than 100 TΩ (1014 Ω) with input bias current below 3 fA, so it can, for example, measure the voltage on a 500 pF capacitor without significantly discharging it, or the potential of piezoelectric crystals and high-impedance pH electrodes.
As an ammeter it is a feedback ammeter: the output voltage is proportional to the input current, and the sensitivity is set by the feedback resistance. A high-gain amplifier holds the input close to zero volts, so the voltage burden (the voltage the meter itself drops in the current path) stays tiny. Keithley states that the voltage burden of an electrometer ammeter or picoammeter is less than 1 mV. It describes the electrometer as able to detect currents as low as 1 fA (10-15 A).
The four functions and their stated limits
- Voltmeter: input resistance typically above 100 TΩ, input bias current under 3 fA.
- Ammeter: currents as low as 1 fA, suited to ion-beam current, photodiode dark current, gate leakage current and mass-spectrometer output.
- Ohmmeter: by the constant-current method, measurements up to 200 GΩ; by the preferred constant-voltage method (apply a known voltage, measure the current, calculate resistance), up to 10 PΩ (1016 Ω). The constant-voltage method is used for insulator resistivity and the insulation resistance of cables, connectors and printed circuit boards.
- Coulombmeter: current integration and charge measurement, with charge detectable down to 10 fC. Keithley notes this is not a capability of multimeters, and that it can be used with a Faraday cage to measure charge on objects.
Electrometer vs multimeter: what is the difference?
A multimeter is a general-purpose instrument designed for conventional measurements; an electrometer is designed to work near the theoretical limits of low-level measurement. Keithley says a DMM generally has a high input voltage drop (burden) that affects low-current measurements and a resolution generally no better than 1 nA, whereas an electrometer or picoammeter has a much lower burden and better sensitivity. Keithley also notes that the most sensitive DMMs can resolve DC currents down to 10 pA, detect DC voltages down to 10 nV and measure resistance from 100 μΩ to 1 GΩ, still well short of an electrometer.
Comparison of low-level DC instruments (Keithley handbook, 7th ed.)
| Instrument | What the handbook says |
|---|---|
| Electrometer | Four functions (voltage, current, resistance, charge); input resistance typically above 100 TΩ; currents down to 1 fA; resistance up to 10 PΩ; charge down to 10 fC. Optimised for high source resistances. |
| Picoammeter | An ammeter built along the lines of the electrometer’s ammeter function: similar low voltage burden, similar or faster speed, less sensitivity. May add features such as logarithmic response or a built-in voltage source. |
| Digital multimeter (DMM) | General purpose; resolution generally no better than 1 nA; most sensitive models resolve DC current to 10 pA and read resistance up to 1 GΩ. |
| Nanovoltmeter | Optimised for voltage measurement from low source resistances; voltage noise and drift much lower than an electrometer, current noise and drift much higher; input resistance usually similar to a DMM; sensitivity from about 1 nV down to a few picovolts. |
| Source measure unit (SMU) | Sources and measures voltage and current; some have electrometer-like characteristics, with input resistance typically 100 TΩ or more and current sensitivity typically as low as 10 fA. |
Electrometer vs picoammeter
Both read very small currents with a very small voltage burden. Per Keithley, the picoammeter trades away some sensitivity (and the electrometer’s voltage, resistance and charge functions) for a narrower, often faster instrument. If the job is current only, a picoammeter may be enough; if you also need high-impedance voltage, insulation resistance or charge, the electrometer’s multiple functions apply. Picoammeters and electrometers are often described in prefixes of pico (10-12), femto (10-15) and atto (10-18) amperes.
What is the instrument that measures electric current?
An ammeter measures electric current. Multimeters include an ammeter range. For currents in the picoampere to femtoampere range, an electrometer (in its ammeter function) or a picoammeter is used instead, because a conventional meter’s voltage burden and bias current would distort such a small current.
What are the different types of electrometers?
The handbook does not divide electrometers into numbered classes. In practice the useful distinctions are by function and by how the current is sensed:
- Voltmeter-type use for high-impedance sources such as pH electrodes and piezoelectric crystals.
- Feedback ammeter or picoammeter use for tiny currents, where a high-gain amplifier keeps the voltage burden near zero.
- Ohmmeter use in constant-current or constant-voltage mode for very high resistances.
- Coulombmeter use (a feedback integrator) for charge, including static charge collected in a Faraday cage.
Where it matters in the lab: an error-source checklist
At these levels the setup matters as much as the meter. Keithley’s handbook names these error sources and responses:
- Input bias and offset current. Check by placing the supplied metal cap on the input (triax or BNC) after a one-hour warm-up in a stable environment, on the lowest current range. The input should be left open, not shorted.
- Do not test a feedback ammeter by shorting its input the way you would a voltmeter.
- Leakage current in cables, fixtures and insulators. Guarding reduces it; triax cabling with a driven guard is used for low-noise work.
- Humidity and contamination. Insulators absorb differing amounts of water; ionic contamination can produce spurious current, especially in high humidity. Take measurements in an environmentally controlled room where possible.
- Triboelectric and piezoelectric currents from flexing cables and stressed materials.
- Fixture insulation. Periodically check fixtures’ insulation resistance with an electrometer ohmmeter.
Instruments used for regulated or accredited work also need a documented calibration; see what calibration is, how to set a calibration interval, calibration certificates and traceability and what to do when a check fails. For another high-impedance electrode instrument, see what a pH meter is.
Common misunderstandings
- “An electrometer is just a better multimeter.” It is optimised for a different regime (high source resistance, tiny currents). A nanovoltmeter, not an electrometer, is the instrument optimised for low source resistance and very small voltages.
- “More sensitive means more accurate.” Leakage, bias current, humidity and cabling can dominate the reading long before the meter’s own limits do.
- “A picoammeter and an electrometer are the same thing.” A picoammeter is the ammeter function alone, with somewhat less sensitivity.
FAQ
What is an electrometer?
A high-input-resistance DC instrument that measures very small voltages, currents, resistances and charges with minimal loading of the source.
How does an electrometer work?
Its input has very high resistance and very low bias current, and its ammeter and coulombmeter functions use a high-gain feedback amplifier so the voltage burden stays near zero.
What is the difference between a multimeter and an electrometer?
A multimeter is general purpose, with current resolution generally no better than 1 nA and a higher voltage burden. An electrometer reaches 1 fA, has input resistance above 100 TΩ, and adds a charge function.
What are the different types of electrometers?
See the function-based breakdown above; related instruments are the picoammeter, nanovoltmeter, DMM and source measure unit.
What is the instrument that measures electric current?
An ammeter; for extremely small currents, a picoammeter or an electrometer’s ammeter function.
Sources
- Keithley (Tektronix), Low Level Measurements Handbook, 7th edition, sections 1.3 and 2: download.tek.com/document/LowLevelHandbook_7Ed.pdf. Read 2026-11-07. A manufacturer handbook; figures are the manufacturer’s typical or stated values, not universal specifications for every instrument.
