Lyden Jar and Wimshurst Machine

Leyden Jar 1st picture
The Leyden jar was invented in the 1745 and it was was an early device for storing electric charge and it was the first capacitor. Leyden jars were used to conduct many early experiments in electricity.

A typical Leyden Jar design consisted of a top electrode electrically connected by some means (usually a chain) to a metal foil coating part of the inner surface of a glass jar. A conducting foil was wrapped around the outside of the jar, matching the internal coated area. The jar was charged by an electrostatic generator connected to the inner electrode while the outer plate was grounded. The inner and outer surfaces of the jar stored equal but opposite charges.

Benjamin Franklin investigated the Leyden jar and concluded that the charge was stored in the glass, not in the water. The charge was actually stored not in the conductors, but only in a thin layer along the facing surfaces that touch the glass, or dielectric, maybe leaking to the surface of the dielectric if contact is imperfect and the electric field is intense enough. Because of this, the fluid inside can be replaced with a metal foil lining. Experimenters found that the thinner the dielectric, the closer the plates, and the greater the surface, the greater the amount of charge that could be stored at a given voltage.

Further developments in electrostatics revealed that the dielectric material was not essential, but increased the storage capability (capacitance) and prevented arcing between the plates. The two plates separated by a small distance acted as a capacitor.

Wimshurst Machine 2nd picture
The Wimshurst machine was an electrostatic device for generating high voltages developed in 1880-1883 by. Its distinctive appearance has two large contra-rotating discs mounted in a vertical plane, two cross bars with metallic brushes, and a spark gap formed by two metal spheres.These machines belong to a class of generators called influence machines, which create electric charges through electrostatic induction, or influence.

In a Wimshurst machine, the two insulated disks and their metal sectors rotate in opposite directions passing the crossed metal neutralizer bars and their brushes. An imbalance of charges is induced, amplified, and collected by two pairs of metal combs with points placed near the surfaces of each disk. These collectors are mounted on insulating supports and connected to the output terminals. The positive feedback increases the accumulating charges exponentially until the dielectric breakdown voltage of the air is reached and a spark jumps across the gap.

The machine is self-starting which means that external electrical power is not required to create the initial charge. But the machine does require mechanical power to turn the disks against the electric field, and it's this energy that the machine converts into electric power. The output of the Wimshurst machine is a constant current that is proportional to the area covered by the metal sectors and the rotation speed. The insulation and the size of the machine determines the maximum output voltage that can be reached.