Lets see if I have this right: A voltmeter has black and red leads. Connecting it to a battery with black on - and red on + will read, say, +9v. This is because there is a net surplus of electrons with charge on the black lead and a net deficit of electrons on the red lead. The pd causes current to flow, charge moving from - to + on the battery. My reasoning is the polarity on the voltmeter must indicate the direction of current flow. But is this always the case? Are there cases where a pd is +3v, but charge is flowing red to black?