We never trusted our eyes with electricity. It does not show. So we built a small box that listens for it and answers in numbers. Learn to ask it the four questions.
⟨ transmission ends ⟩By the end of this page you will take the 4 readings a bench needs: how many volts a part is getting, how many milliamps run through a loop, how many ohms a part has, and whether a wire is whole. You will also blow the meter’s fuse once, on purpose, here where it costs nothing, so you never do it to a real one.
The instrument is the multimeter: one box, one dial, two leads. The black lead lives in the socket marked COM. The red lead sits in the socket marked VΩ for volts and ohms, and moves to the mA socket for current. The dial chooses the question.
The first question is volts, and it is the gentle one. Voltage is measured between 2 points, so you touch the probes to both sides of a part while the loop keeps running. On volts the meter lets almost nothing through, and the loop does not notice it. Below is the loop from First Light, lit. Touch the probes across the LED.
9 V battery, 470 Ω, red LED. The LED keeps 1.90 V, the resistor 7.07 V, and the battery gives 8.97 V at its terminals: its 2 Ω inside keep the last 0.03 V while 15.0 mA flows. On volts this meter is a 10 MΩ path (M is mega, a million: 10 million ohms, not to be confused with the small m of milli), so the loop does not change. A probe touching nothing shows — —; a real meter shows a small wandering number instead.
Red on the higher side gives a plus sign; swapped, the same number with a minus. And the shares add up: 1.90 V for the LED and 7.07 V for the resistor make the 8.97 V the battery gives. A loop always splits its source this way.
Current is different. Amps are a flow, and a flow can only be counted by making it pass through the meter. On mA the meter is not a listener but a piece of wire with a counter inside. So the loop must be opened and the meter put in the gap. The meter below already has its red lead in the mA socket. Pull the jumper out, then probe into the gap.
In the gap the meter reads 15.0 mA, current entering the red probe. On mA this meter is a 0.6 Ω path, almost a short. Dropped across the LED without opening the loop it steals the LED’s current and reads 19.0 mA. Across the battery or the resistor it takes 2.7 A or more and blows its fuse, which the last instrument does on purpose. Model: a 9 V battery with 2 Ω inside, which is why a short through the meter draws 3.5 A and not infinity.
A loop carries the same current at every point, so the gap can be anywhere in it and the number is the same. Ohms are the third question, and the rule is strict: on the ohm setting the meter makes its own small current, pushes it through the part, and works the resistance out from the drop. Any other current in the part ruins the count, and a live loop can push enough into the meter to damage it. So: power off, and the part alone if you can. Open the switch, then probe across the resistor.
Switch closed, the ohm input meets the loop’s own 7.07 V and the display shows Err. Switch open, the resistor reads 470 Ω. The LED reads OL on this setting: it is not a resistor, and the meter’s small test current says nothing useful about it.
The beep setting is the ohm setting in a hurry. Instead of settling on a number, the meter beeps the moment its test current gets through. That is enough for the most common bench question: is this wire whole? Same rule: no power in the thing you test. One of the 3 wires below is broken inside its insulation, where no eye can see it.
Three jumpers; the middle one’s copper is broken under the insulation. A whole wire reads a fraction of an ohm and beeps; the broken one reads OL and stays silent. The page plays a soft beep too, if your sound is on.
One mistake blows more meters than any other: the dial on mA, the probes across a battery or a supply. On mA the meter is nearly a short, so the battery pushes everything it has through it. A fuse inside the meter melts to save the rest. Do it here, once, and remember the feeling.
A fresh 9 V alkaline, about 2 Ω inside, through the meter’s 0.6 Ω: about 3.5 A, 9 times the 400 mA fuse. The melt is slowed to watch; a real fuse goes in a fraction of a second. Afterwards every current range reads 0 until the fuse is replaced; volts and ohms still work.
Now meet the instrument properly: what it looks like on the bench. A meter is a tool, not a part; it is never drawn in a circuit.
When you are ready, test yourself. Four situations from real benches:
You want the current through an LED in a running loop. Where does the meter go?
Since yesterday your meter reads 0.00 on every current range, but volts read fine. What happened?
A jumper wire in your build might be broken inside its insulation. The fastest check?
You want to measure a resistor that is wired into a board. What comes first?
You can now ask the bench what it is doing and trust the answer. The next fragment, The Breadboard, gives you the board those answers are taken on: which holes are joined inside, and the habits that keep a build easy to probe.
Lesson 4 done.
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