| |
LW1DSE > TECH 13.01.18 22:13l 148 Lines 7942 Bytes #999 (0) @ WW
BID : 1131-LW1DSE
Read: GUEST
Subj: The Leyden's Jar
Path: IW8PGT<IR2UBX<EA2RCF<LU9DCE<LU1DBQ<LU7DQP
Sent: 180113/1928Z @:LU7DQP.#LAN.BA.ARG.SOAM #:27252 [Lanus Oeste] FBB7.00i
From: LW1DSE@LU7DQP.#LAN.BA.ARG.SOAM
To : TECH@WW
[――― TST HOST 1.43c, UTC diff:5, Local time: Mon Nov 20 18:56:37 2017 ®®®]
The Leyden jar, or Leiden jar, is a device that "stores" static
electricity between two electrodes on the inside and outside of a jar. It was
invented independently by Ewald Georg von Kleist on 11th October 1744 and by
Pieter van Musschenbroek in 1745~1746. The latter place of invention, Leiden
in the Netherlands gave the invention its name. It was the original form of
the capacitor. The Leyden jar was used to conduct many early experiments in
electricity, and its discovery was of fundamental importance in the study of
electricity. Previously, researchers had to resort to insulated conductors of
large dimensions to store charge. The Leyden jar provided a much more compact
alternative.
Contents
* 1 Description
* 2 History
* 3 The "dissectible Leyden jar" myth
Description
A typical design consists of a glass jar with conducting metal foil
coating the inner and outer surfaces. The foil coatings stop short of the
mouth of the jar, to prevent the charge from arcing between the foils. A rod
electrode projects through the mouth of the jar, electrically connected by
some means (usually a chain) to the inner foil, to allow it to be charged.
The jar is charged by an electrostatic generator, or other source of electric
charge, connected to the inner electrode while the outer foil is grounded.
The inner and outer surfaces of the jar store equal but opposite charges.
Cross-section diagram from about 1900 showing construction.
The original form of the device was just a glass bottle partially
filled with water, with a metal wire passing through a cork closing it. The
role of the outer plate was provided by the hand of the experimenter. Soon it
was found that it was better to coat the exterior of the jar with metal foil
(Watson, 1746), leaving the (accidentally) impure water inside acting as a
conductor, connected by a chain or wire to an external terminal, a sphere to
avoid losses by corona discharge. It was initially believed that the charge
was stored in the water. Benjamin Franklin investigated the Leyden jar, and
concluded that the charge was stored in the glass, not in the water, as
others had assumed. In general the charge may be stored in the conductors, on
the surface along the inward surfaces, or on the surface of the dielectric if
these are separated by a thin air gap. The charge leaks 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.
Early 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 wasn't essential, but increased the storage capability (capacitance)
and prevented arcing between the plates. Two plates separated by a small
distance also act as a capacitor, even in vacuum.
Measuring Leyden jar.
Originally, the amount of capacitance was measured in number of 'jars'
of a given size, or through the total coated area, assuming reasonably
standard thickness and composition of the glass. A typical Leyden jar of one
pint size has a capacitance of about 1 nF.
History
The ancient Greeks already knew that pieces of amber could attract
light particles after being rubbed. The amber becomes electrified by tribo-
electric effect, mechanical separation of charge in a dielectric. The Greek
word for amber is "elektron" and is at the origin of the word "electricity".
Around 1650, Otto von Guericke built a crude friction generator, a
sulphur ball that rotated on a shaft. When Guericke held his hand against the
ball and turned the shaft quickly, a static electric charge built up. This
experiment inspired the development of several forms of "friction machines",
that greatly helped in the study of electricity.
The idea for the Leyden jar was discovered by both German scientist
and jurist Ewald Georg von Kleist, and by two Dutchmen Musschenbroek and
Cunaeus. It was in 1744 when von Kleist lined a glass jar with silver foil,
and charged the foil with a friction machine. Kleist was convinced that a
substantial electric charge could be collected when he received a significant
shock from the device. The effects of this Kleistian jar were independently
discovered around the same time by Dutch scientists Pieter van Musschenbroek
and Cunaeus at the University of Leiden. Musschenbroek communicated on it
with the French scientific community where it was called the Leyden jar.
Daniel Gralath was the first to combine several jars in parallel into
a "battery" to increase the total possible stored charge. By the middle of
the 19th century, the Leyden jar had become common enough for writers to
assume their readers knew of and understood its basic operation. By the early
20th century, improved dielectrics and the need to reduce their size and
inductance for use in the new technology of radio caused the Leyden jar to
evolve into the modern compact form of capacitor.
The "dissectible Leyden jar" myth
A popular, but possibly misleading, demonstration with a Leyden jar
involves taking one apart after it has been charged and showing that the
energy is stored on the dielectric, not the plates. The first documented
instance of this demonstration is in a 1749 letter by Benjamin Franklin.
The jar in the demonstration is constructed out of a dielectric cup
nested between two fairly snugly fitting metal cups. When the jar is charged
with a high voltage and carefully dismantled, it is discovered that all the
parts may be freely handled without discharging the jar. If the pieces are
re-assembled, a large spark may still be obtained.
When not properly explained, this promotes the myth that capacitors
store their charge inside their dielectric. However this phenomenon is a
special effect caused by the high voltage on the Leyden jar. In capacitors
generally, the charge isn't stored in the dielectric, but on the inside
surfaces of the plates, as can be seen from the fact that capacitors can
function with a vacuum between their plates. In the dissectible Leyden jar,
charge is transferred to the surface of the dielectric cup by corona
discharge when the jar is disassembled; this is the source of the residual
charge after the jar is reassembled. Handling the cup while disassembled
doesn't provide enough contact to remove all the surface charge.
O Inner foil
³
______³______
\ ³ /
\ ³ /
±\ ³ /± Fig 1: Diagram of a Leyden Jar.
± \__³__/ ±
± ³ ±
³±³ ³ ³±³
³±³ ³ ³±³
³±³<ΔΔΩ ³±³
³±³ ³±³
³±³ ³±ΓΔΔΔΔΔΔo External foil
³±³ ³±³
³±³ ³±³
³±³ ³±³
³±³ ³±³
³±³ ³±³
± ±
± ±
±±±±±±±±±±±
ΙΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝ»
Ί Osvaldo F. Zappacosta. Barrio Garay (GF05tg) Alte. Brown, Bs As, Argentina.Ί
Ί Mother UMC ζPC:AMD486@120MHz 32MbRAM HD SCSI 8.4Gb MSDOS 7.10 TSTHOST1.43C Ί
Ί 6 celdas 2V 150AH. 18 paneles solares 10W. Ί
Ί lw1dse@yahoo.com ; lw1dse@gmail.com Ί
ΘΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΌ
Read previous mail | Read next mail
| |
