 In this video, we're going to be talking about the effective value or the RMS value of a sine wave. That's very important to us because that is the actual value that we use when we're measuring on an AC voltmeter, so it's the actual voltage. When we talk about 120 volts at your plug or your receptacle or whatever you want to call that, we're talking about the effective value or the RMS value of that. Now effective voltage is defined as the AC value that will give the same heating effect as the equivalent DC voltage. Let me dig a little bit into that. So let's say back in the day before Edison came along, or sorry, before Tesla came along, Edison is running the show here and we have 120 volts and we know that it gives us a certain current and with this resistor here, it dissipates 720 watts. Within Tesla comes along, it gives us our alternating current and it's not so easy to define because this 120 volts with this DC battery is steady. It is always just 120 volts, nothing's changing. But when we add the 120 volts to it, what we have something that's constantly cycling from a zero value to a peak value back down to a zero value to a peak value and so on and so on, it's constantly changing. So what they did is basically they put a guy into a little room, had a heater, hooked it up to 120 volts DC, put his hands close to it, figured out how hot that was. Then took the DC battery out and put an AC power source in an alternator and started cranking it up until he felt the same heat that he did over here. And then they called that 120 volts AC. And while that story might not be true, and it isn't, that kind of gives you the idea of what I mean though. It's, they call this 120 because it's constantly changing, but it's when we reached that 720 watts that we would have reached at this 120 volts DC. Then we just called it 120 volts AC. So RMS you'll hear is roots mean squares. It is determined by taking the square root of the arithmetic mean of the squares of the values. That's a lot of fancy stuff to say, go Google it. If you're totally wanting to know more about what RMS is for our purposes here, we're just trying to figure out a little bit about the arithmetic cycle in this wave form. Don't get too hung up on this whole what this RMS is. If you want to dig deeper, there is something called Wikipedia or Google for that. Now this is the formula that we use and you'll notice that there's a common theme along here is if we take the peak value, we always start with the peak. Now when I was in second year electrical school, I had an instructor that always told me that anytime you wanted to find out something you needed, wanted to see everything, you go to the top of the mountain and from there you could calculate everything you needed to see. So which has stuck with me for 20 some odd years here because if I'm at the peak here, I can take peak times 0.637 to get average and I can take peak times 0.707 to get our effective or our RMS value. So let's say that I have a peak value of current of 31.7 amps and I wanted to figure out what my effective value of current is, my RMS value or what my AC meter would read. I would just take 31.7 times 0.707 to get 22.4 amps and that would be my effective value. That's the value that my AC ammeter would read if I had it hooked up to this circuit.