# Do not throw this toy out for now – take a multimeter

The first time I used a multimeter in my youth. I really liked remote-controlled cars, but you could not buy one and start driving. You had to buy a kit and a controller separately and put it all together. My car was powered by a battery and an electric motor instead of more complex gasoline engines. So, whenever things did not work out, I had to figure out how to use a multimeter to measure the voltage at different places in the car.

Of course, there have been many other cases where the multimeter has proven useful, but let 's just say that it is a great tool. We also use the multimeter in introductory labs to physics. Many students have no experience with these devices, but it does not matter. I will give you (and them) an elementary introduction to the measurement of the electric current and the difference of electric potential (usually called "voltage").

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

It's pretty simple. The electric current comes out of the positive terminal of the battery (OK, in fact, it comes out of the negative terminal) and goes through the lamp and then back to the battery. That's your electrical 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 several things. It can measure both current and voltage. Many multimeters can even do more than that – measure resistance, capacitance, and so on. But let's move on to that. The key to measuring the electrical current is that the multimeter must be on the path of the current. In fact, you have to dismantle your circuit and insert the multimeter into the path.

You must also make sure you are using the right settings and connectors. Most multimeters will have a switch to put it in "ammeter" mode and special plugs to measure the electrical current. Check out this very old (awesome) analogue meter next to a modern digital multimeter.

Note that both use the same plugs for voltage and current, but you should always have the setting on something that reads a current. For the old counter on the left, I set it on a maximum current of 750 milliamps and the digital on a maximum current of 200 mAmp.

Now let's add this to the battery-bulb circuit. Here is what it looks like.

Let's follow the wiring to make sure we know what's going on. Oh, a quick warning. This is an old analog meter. It measures only the electric current in one direction. If you hang it in the wrong direction, the needle will be "fixed" to zero; it is not helpful and it could possibly bend the needle. Be careful.

If you start on the positive side of the battery, the wire goes to the bulb and then comes out of the bulb. Then it goes to the positive terminal of the multimeter, then the negative terminal to the negative terminal of the battery. Like a regular battery and a light bulb circuit, it forms a complete loop. You can simply consider the ammeter as a wire that connects the bulb to the battery. It is a wire that also measures the current.

Well. So, how do you measure the tension? You must connect the multimeter differently to measure the voltage. Here's something to remember: you measure the tension through something, you measure the current through Something. So, to measure the voltage, you have to place the two multimeter probes on both sides of the thing you want to measure. Let's measure the voltage across the bulb. I will put the circuit back as it was (before measuring the current) then touch a probe on either side of the bulb. Here is what it looks like.

If you follow the wires for this configuration, you can see that the original circuit is still there. The multimeter leads should not break the circuit but simply touch both ends of the bulb. Oh, be sure to swap the multimeter settings to read the voltage instead of the current.

It is important to realize that the voltage and current readings are connected in different ways. In fact, this is the # 1 error made by students when using this instrument. This is what happens. They want to measure the current, so they have all the parameters and all the multimeter plugs configured for the current. Then they touch a multimeter lead from one side of the light bulb and the other from the other (the way you would use it to measure the voltage). Blam-o. You have just blown a fuse. Why? With the multimeter installed in front of the bulb, you have placed the indicator in parallel with the bulb. As the ammeter setting has low resistance, you get a mass of electrical current flowing through the meter. You basically measure the electrical current through a wire containing nothing. Of course, the meter can only handle as much current, so you blow up a fuse.

Now the counter is not working. You must replace the fuse. At least you have not messed up with the multimeter, it would be a shame.