#63 - Sarah S.

Wimshurst

Leiden

The Leiden Jar
History: 1745 - Ewald Kleist, stored large amounts of electric charge by lining a glass jar with silver foil, and charged the foil with a friction. He received a shock. 1746 - Dutch physicist Pieter van Musschenbroek of the University of Leyden made the same discovery
Consists of: A glass cylindrical container (jar) with an outer and inner metal (foil) coating covering the bottom and sides of an insulator (plastic or glass).
Brass rod with an external knob passing through a wooden stopper that is connected to the inner coating by a loose metal chain.
How it works: an electrical charge is applied to the external knob and positive and negative charges accumulate from the two metal coatings respectively. However, they are unable to discharge due to the glass between them. The result is the charges will hold each other in equilibrium until a discharge path is provided. Charge is stored not in the conductors but in a thin layer along the facing surfaces that touch the glass. When the outside and inside surfaces are connected by a conductor there is a spark and everything is grounded.
Uses: store electricity in experiments and a condenser in early wireless equipment.

The Whimshurst Machine
History: developed between 1880 and 1883 by James Wimshurst
Consists of : with 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.
How it works: creates electric charges through electrostatic induction
two insulated disks and their metal sectors rotate in opposite directions, metal foil sectors on the disks induce charges on each other, which are amplified and collected by metal brushes and stored in Leiden jars, spark jumps across gap
machine is self-starting (no external electrical power required to create the initial charge). Does require mechanical power to turn the disks against the electric field (this is the energy that the machine converts into electric power). The output is a current proportional to the area covered by the metal sectors and to the rotation speed.