CourseWiki/Engineering Fundamentals/Electrical Systems

Measuring Electric Circuits

Connect voltage, current, resistance and continuity measurements to the part of the circuit they describe.

Voltage is measured across; current is measured through

A voltmeter measures the potential difference between two points. Connect it across the component, in parallel. A series ammeter becomes part of the current path: open the path and insert it in series with the element whose current you want.

A DC source supplies an ammeter and resistor in series. A voltmeter connects across the resistor, on two separate parallel leads.

A meter set for current has a low-resistance input; connecting that input across a source makes a short circuit. Read Electrical Safety before making physical measurements. The diagrams here explain connections, not a procedure for working on mains wiring.

Real meters change the circuit a little

An ideal voltmeter draws no current. A real one has high but finite input resistance. An ideal ammeter drops no voltage. A real one has some resistance and a voltage drop, sometimes called burden voltage. The practice problems use ideal meters unless stated otherwise.

Place both voltage probes, predict the sign, then reveal; Interactive; Practice

Place both voltage probes, predict the sign, then reveal

A DC voltmeter reports an ordered difference: V(red) − V(black). The two probes must touch the two named points you want to compare. Reversing them reverses the sign; putting both on the same conductor gives zero.

Static example: an ideal 12 V source supplies 30 Ω and 60 Ω resistors in series. Each resistor has an ideal switch immediately before it.

Signed readings from the same ideal circuit
Circuit state	Red point	Black point	Reading	Why
S1 open	Supply +	After S1	+12 V	The single open switch takes the source voltage; no current flows.
Both closed	After S1	After R1	+4 V	Current is 12/(30+60) = 0.1333 A; R1 drops 4 V.
Both closed, probes reversed	After R1	After S1	−4 V	The two point potentials are unchanged; only the ordered subtraction reverses.
S2 open	After R1	After S2	+12 V	The upstream side stays at +12 V and the downstream side returns to 0 V.
Any supported state	One named point	The same point	0 V	A point cannot have a potential difference from itself.

With two open switches, the conductor between them floats in this ideal model. Its voltage is not assigned. The interactive intentionally offers no two-open state.

This is a low-voltage ideal-circuit model with an infinite-impedance voltmeter. It does not authorize live work, mains measurements, or current-mode probing. Fluke's DC-voltage guide explains the V and COM inputs and the negative sign from reversed DC probes; its 381 meter manual shows that voltage probes touch the two desired test points.

Practice choosing probe points Practice open-switch readings

A shunt or clamp lets you measure current indirectly

A shunt is a known small resistance in the current path. Measure the voltage across it and use the relationship in Current, Voltage, and Resistance to find current. The shunt is in series; the voltage measurement across it is in parallel. Many ammeters contain a shunt internally. An installed shunt avoids opening the circuit each time you read it; adding one still requires changing the path.

A clamp meter senses the magnetic field around a conductor without breaking the circuit. Clamp around one conductor, not the outgoing and returning conductors together: their fields can cancel. The instrument must support the type of current being measured; an AC-only clamp does not measure steady DC.

Resistance and continuity are measured with the circuit unpowered

An ohmmeter applies its own small test signal. Use it on an unpowered circuit, with stored energy discharged as described in Electrical Safety. Other connected paths can change the result; isolating the component lets you measure its resistance instead of the whole network.

Continuity is a resistance-threshold test. The beep means the measured resistance is below that meter's threshold. It does not prove exactly zero resistance, identify the intended path, or establish that a connection can carry its working current. The threshold comes from the meter's manual.

A multimeter combines voltage, current and resistance functions, often with continuity too. Select the function, AC or DC mode, range and lead sockets for the measurement. In a schematic, follow the actual connection points rather than the physical positions of the symbols.

Appears in

Circuits, AC and DC DET 403 DET