 A voltmeter is used to measure a potential voltage difference across two points in a circuit. To measure voltage in a circuit, the meter is connected in parallel with the components to be measured. To accurately measure voltage, it's important that the meter have a negligible impact on the circuit being measured. Therefore, voltmeters have a high internal resistance to prevent current from flowing through the meter, connecting them to a component therefore will not affect the voltage being measured. If the voltmeter's internal resistance is less than 20 times the components resistance, current will flow out of the circuit into the meter, resulting in an inaccurate voltage reading. Ammeters are used to measure current in a circuit. They are connected in series with the components to be measured. This means that the ammeter is placed in line with the circuit to be measured. Ammeters have a low internal resistance so that connecting them in series with the component should not change the resistance of the circuit and therefore not affect the current being measured. Never connect an ammeter in parallel with a component. Because of the very low resistance of the ammeter, the current from the circuit will bypass the resistor and move through the meter. If the current level is high and there are no other current limiting components in the circuit, the high current flow can damage the meter. To measure resistance of a circuit component, an ohm meter is used. Like a voltmeter, an ohm meter is connected in parallel with the component to be measured. Ohm meters are never to be used in a powered circuit. It can damage the meter as well as present a safety hazard. So make sure switches are open before applying the leads of an ohm meter to a circuit. To measure resistance, an ohm meter has an internal voltage source which generates a small current that flows through the component being measured. The meter will automatically perform an ohm's law calculation by dividing the voltage it applies by the current flowing resulting in the resistance value. Today, all-in-one digital multimeters like this can be used as a voltmeter, ohm meter, or ammeter. They offer faster and more accurate readings.