Voltmeters:

Voltmeters are used to measure emf's and more commonly potential differences. The two probes of the meter are therefore connected to the two points between which the potential difference is required.

If the potential at A with respect to B is required, the red lead is connected to point A, the black lead point B.
If the potential at B with respect to A is required, the red lead is connected to point B, the black lead point A.
If the potential between a point and Earth or ground is required. The red lead is connected to the point and the black lead is connected to ground or Earth.