The measurement is generally conducted using a three-electrode setup (see the drawing):
# working electrode,
# counter electrode,
# reference electrode (standard hydrogen electrode or an equivalent).
In case of non-zero net current on the electrode, it is essential to minimize the ohmic IR-drop in the electrolyte, e.g., by positioning the reference electrode near the surface of the working electrode (e.g., see Luggin capillary), or by using a supporting electrolyte of sufficiently high conductivity.  The potential measurements are performed with the positive terminal of the electrometer connected to the working electrode and the negative terminal to the reference electrode.