Actual electric potential at terminals of battery
Potential is always measured relative to some reference point. This can be the earth, the moon, the car chassis, the negative or positive terminal of the power source or even to an AC signal. For most practical applications we don't use infinity as a reference but rather something much more local.







simulate this circuit &ndash; Schematic created using CircuitLab

Figures 1 to 8. Various measurement and reference schemes for a 9 V battery and voltmeter.


The voltmeter measures the battery voltage as there is a complete circuit.
In this case there is a circuit through local earth. The voltmeter shows 9 V.
This represents a chassis connection. Again 9 V.
In this case some arbitrary point in the circuit has been taken as ground. In most battery powered equipment this will be the DC negative supply. One recent question on this site showed an old transistor radio circuit with the battery positive as the "ground".
We have the option of connecting the circuit ground to earth. This prevents floating of the device power supply and might be used for safety, to avoid audio hum, etc., depending on the application.
Without a reference for the voltmeter it will read 0 V. Note that a very sensitive digital meter will show random readings due to stray electric fields. Putting a medium resistance (say 100k) across the meter terminals will cause the reading to collapse to zero.
Inverting the battery we now have a positive ground. The voltmeter will read -9 V.
Inverting the meter so that its positive lead is grounded will also result in a -9 V reading.


For most practical electronics you just need to work out the potential between points. When debugging it is very often most convenient to attach the negative probe to the circuit ground and take all readings with reference to that point.