Phase boundary potentials and double layers

The two sides, positive and negative, of the electrical double layer may be situated on different sides of the material phase boundary, or on the same side. There are three different ways in which electrical double layers may arise, when two phases are brought into contact, and therefore three different origins for interfacial potential differences. 

When this equilibrium has been established, the distribution of charged particles in the two phases is not uniform. The positively charged metal 

The third possible origin for double layers is an orientation of neutral 

1 The terms uncharged particles, or components, will be used to describe particles, or components of a system, which have no excess or deficiency of electrons over the number required for electrical neutrality charged particles or components have an excess or deficiency of one or more electrons. 

It seems probable that electrification by friction, the oldest known method of electrifying bodies, is caused by the sudden separation of the two halves of the double layers set up on contact, when the contact is broken. Helmholtz took this view,1 and though there has been some little controversy over it,2 the view still seems the most probable. Ballo-electricity, or waterfall electricity,3 produced when a mass of water breaks up into small drops, is also probably due to the charge produced when double layers form at the surface of drops. It is closely connected with the electrokinetic or potential ( 8). 

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