Just a quick follow-up on why testing under load is sometimes required.


Here the 6 Ω resistance is the headlight filament.
The battery cables are good and the headlight switch is open.
The voltmeter will read 12 volts.



The additional 6 Ω resistance is a faulty connector on the battery cable.
The voltmeter will still read 12 volts with the headlight switch open.
The good and bad circuits look identical to a voltmeter.


With the headlights ON in the good circuit the voltmeter will still read 12 volts.


But in the bad circuit the voltmeter now reads 6 volts.
That bad connection doesn't reveal itself until you try to force current through it.


An ammeter in series with the load would show 2 amps in the good circuit.


Conversly, the ammeter would only read one amp in the bad circuit.