If we connect a voltmeter between points A and B, we find which of the two points has the higher potential. Suppose that we find that the potential at A is higher than that at B. Then we bring the magnetic needle, mounted on a vertical pivot, up under the corresponding wire, e.g. the upper one. The deflection of the magnetic needle's North pole tells us the direction of flow of current through the wire. For example, if the needle's North pole is deflected towards us from the plane of the paper, the current in this wire is flowing through A from right to left. Hence it follows that the source of current in our example is to the right of A.