 So, as I mentioned in the previous video, when dealing with circuits we don't actually care about the absolute voltage, instead we care about the change in voltage, the difference in voltage between two points. So the way that we measure voltage is using a voltmeter. In my case I have a multimeter here which is acting as a voltmeter. Here I have a 9 volt battery which is connected to two different light bulbs. And the number that's on the multimeter shows what is the difference in voltage between the red lead and the black lead. So this number here tells me the number on the red lead minus the voltage on the black lead. Now I've connected my battery to my two light bulbs. I can use the voltmeter to measure the potential difference between two points in the circuit. So I've asked to take both leads and put them on the same point in the circuit here. So the number is going to give me zero. There's zero volts. Any coolant of charge that moves between those two points loses no energy. However, if I put the probes on either side of one of these light bulbs, like that, now the voltmeter is reading approximately 4.5 volts. That means that when a charge moves from this red lead to the black lead, it's losing 4.5 joules of energy per charge. It would be the same if I had to go with the other one here. So if I measured the potential difference between this point and this point in the circuit, then I'm also reading approximately 4.5 volts. So I can use the two leads of a voltmeter to measure the voltage through the circuit by choosing two points I'd like to measure the potential difference between. If I put the voltmeter and used it to measure the two ends of the battery, then we would see that we are getting 9.1 volts. If this is a 9 volt battery, we'd expect the difference between the two terminals to be approximately 9 volts. So here is a diagram of this circuit. What I'd like to point out is I've represented the 9 volt battery with the normal symbol. But in addition, I've put the plus 9 volts on the positive terminal and zero volts on the negative terminal. This is just a way of indicating how powerful the battery is, or in this case it is a 9 volt battery. So normally we just take the negative terminal to be zero and then we increase the positive terminal by whatever the battery's voltage is. To represent a voltmeter on this diagram, I would use this symbol here. It's a circle with a V in the middle. Then what I'd do is I'd choose the two points in the circuit where I want to measure my voltage. So in this case here, my voltmeter would be measuring the difference in voltage between one side of the light bulb and the other. And this would give us 4.5 volts. If I wanted to measure the potential difference across both the light bulbs, what I would do is connect my voltmeter like this. So now it's measuring the difference in potential between this point in the circuit and that point in the circuit, which would be, in this case, 9 volts. No, it's normally the voltmeter symbol would be drawn in black. But in this case, to make it clear, I've drawn it for you in blue.