# 17.3 Voltmeters, Ammeters, and Terminal Voltage

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## Voltmeters, Ammeters, and Terminal Voltage

VOLTMETERS AND AMMETERS
Voltmeters • A voltmeter measures the potential difference.
• Voltmeters are inserted in parallel with a resistor.
• An ideal voltmeter has infinite resistance.

Ammeters • An ammeter measures current.
• Ammeters are inserted in series with a resistor.
• An ideal ammeter has zero resistance.

TERMINAL VOLTAGE AND INTERNAL RESISTANCE
Terminal
Voltage
V = ε - Ir          ε = emf          r=internal resistance of the battery
Internal
Resistance
A typical battery has an internal resistance that effectively functions as a resistor inline (in series) with the rest of the circuit. This lowers the potential difference across any external resistors (often called load resistors). Subtracting the drop in potential due to the internal resistance from the emf yields the terminal voltage of the battery.