The multimeter: four questions it can answer

A multimeter measures volts across two points, ohms through a part with the power off, continuity as a beep for a path with almost no resistance, and, on some meters, amps in series with the circuit.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

The first meter reading you take that is wrong because of the dial and not the circuit is the one that teaches you this. Ohms on a live circuit gives garbage and can damage the meter; volts on a dead circuit tells you nothing about whether it is broken. Each mode answers one question.

How to think about it

Decide the question first. Is there voltage here: volts, across. Is this part good: ohms, power off and the part disconnected from everything else. Is this wire whole: continuity, power off. Then set the dial, then touch the probes. Never measure resistance on anything that could be live: de-energize, lock it out and prove it dead with the volts setting first.

Worked example

Volts, AC, across the two slots of a receptacle: 121 V

Probes across, circuit live. The push that is there.

De-energize, lock out, test dead: 0 V hot to neutral, 0 V hot to ground

Before any ohms reading. The volts mode proves the circuit dead; the test-before-touch lesson says how to trust that zero.

Ohms across a disconnected 1000 W, 120 V heater element: about 14 Ω

R = V² / P = 120 × 120 / 1000 = 14.4 Ω. A reading near that is a good element; OL (open) is a broken one.

Continuity end to end on a coil of cable: beep, 0.5 Ω

A whole conductor. No beep and OL means it is broken somewhere in the middle.

Your turn

You want to know if a fuse is blown. The power is off and the fuse is out of its holder. Write the mode.

dial:

The trap

Measuring ohms on a part that is still connected to the rest of the circuit. The meter reads every parallel path around it, so a good element beside a shorted one reads low, and a broken element beside a good one reads fine. Disconnect one end.

Practise the multimeter on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.