 In this video, we'll be looking at how to calculate the internal resistance value of a voltmeter. Now voltmeters have a very large resistance in them as they have to sit parallel to a circuit, and we'll discuss that in a later video. What we're looking at right now is how to size this resistor here, because what we see here is that this galvanometer up top here, which is just our basic coil that's got some resistance in the coil, can see a maximum of 50 milliamps. It has an internal resistance of 100 ohms, which means that if you take the internal resistance with the maximum current that it flows, you'll be able to calculate out the maximum voltage that this whole galvanometer can see. So let's say that we are trying to calculate or measure 100 volts. So this is where our leads would connect to. We want to read 100 volts. We're going to calculate first out what the galvanometer is rated for, which will help us determine what size this resistor is. We're going to go ahead and use Ohm's law. 50 milliamps times 100 ohms gets us 5 volts. That means that the maximum that this galvanometer can see is 5 volts, but we're reading 100 volts. So what are we going to do with the rest of that voltage? Using Kirchhoff's voltage law, we're putting a resistor in series that will take the rest of the voltage. So in this case, 100 minus 5 gives us 95 volts over on this side. Now we also need to remember that in a series circuit, our current remains the same. So up here, if we notice that our galvanometer can see a maximum of 50 milliamps, that means that this resistor here will see a maximum of 50 milliamps because it has the same current flowing through it because it is in series. So what we can do to determine this resistance is we can take this 95 volts and divide it by the current, the maximum current that we'll see, 95 volts, 0.05 amps, which is the same as 50 milliamps, and we can determine our resistor value on this. And in this case, that works out to be 1,900 ohms. Generally, you're going to have different size resistors for different voltage settings. So for a 250 volt reading, you'll actually have a different resistor lined up for that. And for a 600 volt, you'd have a different resistor lined up for that. And you would just have a setting that clicks between this one to this one to this one. But it's the same concept. You're taking the voltage that you have here, you're subtracting it from the voltage you're trying to read, then you're taking the full-scale deflection current, dividing it by that voltage that that resistor will see, and you'll determine your resistor value.