Experimentalists have long studied trends in ∆φ. In the 1960s, Huber and Kirk measured a steady decrease in the work function of Al as they dosed the surface with sub-monolayer amounts of water . Later, this same study was extended to Fe, Co, Ni, Pt, Au and Cu surfaces . In all cases, a reduction in the work function was observed as higher doses of water was added to a surface, before leveling off at some saturation coverage. Furthermore, it was found that the value of this saturation in work function reduction was a surface dependent quantity . These studies offered an indication that ∆φ was directly related to both the water coverage and the surface under consideration.