1. The Leyden Jar

2. The Leyden Jar (invented by van Musschenbroek in 1746 in the town of Leyden) was the first means invented of storing electrical charge. It is a glass or earthenware jar coated on the inside and outside (A and B) with tin foil. A central pillar usually with a circular cap allows electrical contact with the inside layer of foil. The device was charged with static electricity at the central pillar

3. +++ ++ -- When the ball is charged positively a positive charge is acquired on the inside of the jar. A negative charge is induced on the outside as electrons run towards the positive charge. The glass provides insulation. Later this insulating medium became known as a dielectric.

5. Batteries of Leyden Jars enabled early electrical experimenters to store the high-voltage output of their frictional machines. Large amounts of energy could be accumulated, the effects of the subsequent violent discharges being of great interest..

6. Abbe Nollet Abbé Nollet persuaded 200 monks to stand in a big circle a mile round. Each monk held one end of a 25 foot iron wire in each hand; his neighbour around the circle in each direction held the other end The circle was broken at one point where the Abbé Nollet stood with a large Leyden Jar battery. When without warning he connected the two monks at the ends of the line to the terminals of the battery without warning, he noticed that to all intents and purposes, the monks all leapt into the air simultaneously! Thus showing that an electrical shock travels at enormous speed

7. Dispensing with the jar, the same effect can be demonstrated with pieces of metal foil separated by an insulating medium. That medium may be air, paper, mica, a vacuum. The better the insulating properties of the dielectric the more charge that can be stored. The diagram shows a capacitor with foil plates and waxed paper dielectric A foil capacitor with mica dielectric

8. The electrolytic capacitor is commonly used in d.c. circuits. The dielectric is polarised and must not be used in a.c. circuits or connected back to front in a d.c. circuit.