The potential difference, or change in electric potential, between two points is measured with a voltmeter. Current flows through a resistor because of a potential difference applied by a battery or power supply. Potential difference is commonly measured in units of volts (V) or millivolts (mV = 10-3 V). Common usage refers to a potential difference relative to ground (0.0 V) as simply the voltage, though it is prudent to call it by its correct name to emphasize the way it is measured. The potential difference across a circuit element is measured by placing the two leads of a voltmeter on the two sides of the element. Look again at the lower two diagrams in Fig. 4. The voltmeter remains "to the side"; its removal will not change the circuit. Notice the difference in ammeter use where the meter becomes an integral part of the circuit being measuring. The removal of a correctly used ammeter would stop current flow in at least part of the circuit by creating an open condition
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