 In this video, we're going to explore how real ammeters and voltmeters are different to the ideal ones that we've been looking at in the course so far. Just as batteries are not perfect and contain some internal resistance, real voltmeters and ammeters are not completely ideal and will slightly change the circuit that they are measuring. Let's consider a simple circuit that consists of an ideal battery and two resistors or light loads in series. If I wanted to measure the current through this circuit, I would put my ammeter in series with the light bulbs as shown here. An ideal ammeter would have zero resistance, however real ammeters have a very low but non-zero resistance. This means that when they're placed in a circuit, they increase the total resistance slightly and therefore would decrease the total current. In most circuits this is not a problem, but if this circuit had an extremely low resistance to begin with, then adding this ammeter could significantly increase the resistance and then significantly affect the measurement. If I wanted to measure the voltage across the second light bulb, instead of the ammeter, I would instead place a voltmeter in parallel with the second resistor or the second light bulb. A voltmeter is designed to have an extremely high resistance, however it is not an infinite resistance and this means that the voltmeter interferes with the circuit slightly. Since the resistor is now in parallel with the voltmeter, the total resistance of the junction will be slightly decreased. This means that the voltage drop across the second light bulb will decrease slightly and the voltage across the first light bulb will increase slightly. And the measured voltage will be slightly lower than if the voltmeter was not in the circuit. So this means that by adding our voltmeter into the circuit, we have interfered with it when we're making our measurement. However, voltmeters have a very high resistance, usually between 10 to the 7 and 10 to the 9 ohms. So as long as the component being measured has resistance at small compared to this big number here, there should be no significant impact on the voltage measured.