 In this video, we're going to be walking through a voltage divider circuit and we're going to talk about the importance of the ground reference. So let me show you what I mean. Here we've got our basic voltage divider circuit. We have four resistors in series with one another. And it looks very much like a series circuit. We've got a negative going there. The current will flow through these points and get over to the positive. The difference is we have these tap off points, A, B, C, D and E. And you notice here that our ground is referenced to the negative side. So this is our zero volt reference point. When we're working with voltage dividers, we need to use this formula up here in the corner. This is our voltage divider formula. The V of R, which is going to be the resistive voltage across each resistor, is your source voltage multiplied by the resistor divided by the total resistance. Because that's basically the ratio. So we're going to figure out what the ratio is of this resistor to the entire resistance. Multiply it by the source voltage and we'll get the resistor voltage. Let's walk through a few of them. So in this case here, you notice I've said that the voltage across the 80,000 ohm resistor is 101 volts. Let me show you how I got that. What we're basically doing is we're going to take our VR, which is the 80,000, so VR is equal to the voltage source, which is 240 volts, times 80k, divided by the entirety of this, so this plus this plus this plus this, the entire resistance, which in this case works out to be 190k. So you do the math, we work out what this ratio is here, multiplied by 240 volts, and we end up with 101 volts. And we're going to do that for each individual resistor here. For this resistor, we just take 60,000 divided by 190,000, we multiply that by 240, and we get 75.8 volts just across this resistor. On this resistor here, we take 30,000 divided by 190,000, we multiply that by 240, and we end up with 37.9 volts. And for our last one, we take 20,000 divided by 190,000, we multiply that by 240, and we end up with 25.3 volts. So now we have our voltage drops across each resistor, and if this is a series circuit and running fully on the way it's supposed to go, that's what we would see across each one. If we put a voltmeter across each one, that's what we would get as well. Now for taking these, and we're measuring the voltage in reference to ground, if I look at A, if I took a meter, let me just draw in a very crude version of a meter, if I take my voltmeter here, and I'm going from here, and I ground out my voltmeter for reading that to ground, this is going to read the entirety, because it's going to put my reference points all the way down to the bottom here. So we end up with 240 volts to ground. If I move that, let me just get rid of my voltmeter here, if I move my voltmeter, and I measure this to ground, so I take my voltmeter here to B, and I ground it, or I measure it to the ground point, I'm going to get, I'm excluding the 101 volts here, so I'm going to take 75.8 plus 37.9 plus 25.3, and I'm going to read 139 volts. From this point here to this point here is 139 volts, and we could do the same thing here. If we wanted to move this lead here, let me just erase this, if I wanted to move this lead, and change that down to this point, to there, I would just be adding up this voltage and this voltage. So we do that, and we're getting our volt drops across there. Let me just get rid of my beautiful drawing of a second here. It will not erase for me. There we go, got the eraser working. So that's how we determine where our, what our voltages are in reference to ground. Now, when we look at these, it's also important that we get our polarities figured out, because once we start moving this ground around, we're going to get different voltages, and the polarity is very important. So what we're going to do is we're going to pop some polarity into the circuit. So here I've gotten through, and I've taken, this is negative, so this is negative, positive, negative, positive, negative, positive, negative, positive. So what we're going to do now is we're going to move this ground, and we're going to see what happens when we start measuring our voltages from different points to ground. So I've popped the ground up to C here. So what we're going to do is I'm going to pop in a voltmeter again, another beautiful drawing of a voltmeter. So I've got my voltmeter here. And if I read from point A to ground, what's going to happen is I'm no longer reading the entirety of this. I'm only reading in reference to the ground, my zero-volt reference point. So it's going to be 101 plus 75.8. I'm going to read 176, let me just write that down because it's so important. 176.8 volts from A to ground. And also, let's take a look at this. From A to ground is here, I pick up 101 volts, because I'm seeing that positive first, and I pick up 75.8, so it's positive. We've got a more positive than negative here. So I get this, and it is a positive 176.8 volts. Now let's change that up. Let's read from the bottom side of it, on the other side of the reference point. Let me just get rid of my voltmeter here and move it off to another area. So let's say my voltmeter is going to read from here point E to ground. And honestly, I could just put the lead right over to the ground point E. It's the same thing. So when I do that, I add up my voltages here, so it's not going to be too difficult. I just go, you know, there's going to be 25.3 volts and 37.9 volts. So 25.3 plus 37.9 gets me a voltage of 63.2. So I'm reading 63.2 volts, but it's negative. And let's see why that's negative. It's negative because I start here and I'm going up to the reference point, which is the ground right here. So if I do that, I see that I'm hitting that negative first before I enter the resistor. So the way I look at it is like I'm dropping off 25.3 volts and I'm dropping off another 37.9 volts. I have a deficiency now of 63.2 volts. So that's why it's important to understand where this reference, the zero volt reference point is and then to have your polarities drawn in. I know in the first few, I didn't have the polarities because I was making a point on just voltages, but when we actually get down to reading the voltages in relation to the ground point, it is so very important to have these polarities worked out and then you start on one side of your lead and you work to the other side. So this is ground, which is the same as this. So that's why we went negative, negative to ground. On this side, we went positive because we saw positive first. Positive, positive, positive to ground. And this will become very, very useful, especially when we get into three wire circuits and start talking about our neutral break and line breaks.