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Science Science / Dot Physics

Don’t Toss That Busted Toy Just Yet—Grab a Multimeter

The first time I used a multimeter was in my younger years. I was really into remote-controlled cars—but you couldn’t just buy one and start driving. You had to buy a kit and a controller separately and put the whole thing together. My car was powered by a battery and an electric motor instead of the more complicated gas-powered ones. So whenever things weren’t working, I had to figure out how to use a multimeter to measure the voltage at different points in the car.

Of course there are many other times the multimeter came in handy, but let’s just say that it’s a great tool. We also use the multimeter in introductory physics labs. Many students have zero experience with these devices, but that’s OK. I’m going to give you (and them) a super basic introduction to measuring electric current and the electric potential difference (usually called “voltage”).

Let’s start with the simplest circuit. A battery, two wires, and a light bulb.

Rhett Allain

This is pretty straightforward. Electric current comes out of the positive end of the battery (OK, actually it comes out of the negative end) and goes through the bulb and then back to the battery. That’s your electric circuit. Simple.

But now we want to measure the electric current. How do we do that? That’s where the multimeter comes in. It’s “multi” because it does multiple things. It can measure both the current and the voltage. Many multimeters can do even more than that—measuring resistance, capacitance, and other stuff. But let’s get to it. The key to measuring electric current is that the multimeter has to be in the path of the current. You actually have to take apart your circuit and insert the multimeter in the path.

You also need to be sure you use the correct settings and the correct plugs. Most multimeters will have a switch to put it in “ammeter” mode and special plugs to measure the electric current. Here, check out this extremely old analog meter (which is awesome) next to a modern digital multimeter.

Rhett Allain

Notice that both of these use the same plugs for voltage and current, but you still need to have the setting on something that reads a current. For the old meter on the left, I have it set to a maximum current of 750 milliamps and the digital is at a max current of 200 mAmp.

Now let’s add this to the battery-bulb circuit. Here’s what that looks like.

Rhett Allain

Let’s follow the wiring to make sure we know what’s going on. Oh, a quick warning. This is an older analog meter. It only measures the electric current in one direction. If you hook it up the wrong way, the needle will get “pegged” at the zero mark—that’s not useful and it could possibly bend the needle. Just be careful.

If you start from the positive end of the battery, the wire goes to the light bulb and then out of the light bulb. Next, it goes to the positive terminal of the multimeter and then the negative terminal goes to the negative end of the battery. Just like a plain battery and bulb circuit, this makes a complete loop. You could just consider the ammeter as a wire that connects the bulb to the battery. It’s a wire that also measures current.

Fine. Then how do you measure voltage? You have to hook up the multimeter in a different way to measure voltage. Here is something to remember: You measure the voltage across something—you measure the current through something. So, to measure voltage you need to put the two probes from the multimeter on the two sides of the thing you want to measure. Let’s measure the voltage across the bulb. I’ll put the circuit back the way it was (before measuring the current) and then touch one probe to each side of the bulb. Here’s what it looks like.

Rhett Allain

If you follow the wires for this setup, you can see that the original circuit is still there. The wires from the multimeter don’t have to break up the circuit but are instead just touching the two ends of the bulb. Oh, be sure to switch the multimeter settings to read voltage instead of current.

It’s important to realize that the voltage reading and the current reading are hooked up in different ways. In fact, this is the No. 1 mistake that students make when using this instrument. Here’s what happens. They want to measure the current, so they have all the settings and plugs on the multimeter set up for current. Then they touch one wire from the multimeter to one side of the bulb and the other wire to the other side (the way you would use it to measure voltage). Blam-o. You just blew a fuse. Why? With the multimeter set up across the bulb, you have put the meter in parallel with the bulb. Since the ammeter setting has low resistance you get a BUNCH of electric current going through the meter. You are essentially measuring the electric current through a wire with nothing in it. Of course the meter can only handle so much current, so you blow a fuse.

Now the meter doesn’t work. You need to replace the fuse. At least you didn’t mess up the whole multimeter—that would be bad.

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