Answer:
battery
Step-by-step explanation:
Any source of electricity, the battery, or the power supply is there to maintain the potential difference across a conductor.
To measure this difference we use VOLTMETER - Instrument for measuring potential differences, voltage differences. It is called a voltmeter as it allows the measurement of the potential difference or voltage that exists between two points belonging to an electrical circuit. The voltmeter, therefore, detects the voltage.