The Electrophorus took the place of the Leyden jar and is the two-plate principle that is behind the electrical condensers in use today. Invented in 1775 by Alessandro Volta, its purpose was to create and store an electrostatic charge.
What does it consist of? The electrophorus consists of a dielectric plate (originally a 'cake' of resinous material like pitch or wax, but in modern versions plastic is used) and a metal plate with an insulating handle.

The Van de Graaff generator was invented in 1929 by Robert J. Van de Graaff and is one of the most famous of all the electrostatic devices. It uses a conveyor belt to carry an electric charge from a high-voltage supply to a hollow ball. Another generator was modified to produce x-rays to be used in treating internal tumors in Boston in 1927. Van de Graaff's first generator operated at 80,000 volts and eventually would be improved to five million volts. This generator remains one of the most widely used experimental exhibits in schools and museums today.
Why did it come about? Van de Graaff needed to accelerate subatomic particles to a very high velocity to test the properties of atoms. He knew that by storing an electrostatic charge, could result in many benefits. -DH