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ELECTROSCOPEAn electroscope is an early scientific instrument that is used to detect the presence and

magnitude of electric charge on a body. It as the first electrical measuring instrument. The first

electroscope! a pi"oted needle called the versorium! as in"ented by #ritish physician $illiam%ilbert around &'((. The pith)ball electroscope and the gold)leaf electroscope are to classical

types of electroscope that are still used in physics education to demonstrate the principles of

electrostatics. A type of electroscope is also used in the *uart+ fiber radiation dosimeter .Electroscopes ere used by the Austrian scientist ,ictor -ess in the disco"ery of  cosmic rays.

Electroscopes detect electric charge by the motion of a test obect due to the Coulomb

electrostatic force. Since the electric potential or "oltage of an obect ith respect to ground

e*uals its charge di"ided by its capacitance to ground! an electroscope can be regarded as a crude"oltmeter . -oe"er! the accumulation of enough charge to detect ith an electroscope re*uires

hundreds or thousands of "olts! so electroscopes are only used ith high)"oltage sources such as

static electricity and electrostatic machines. Electroscopes generally gi"e only a rough!

*ualitati"e indication of the magnitude of the charge/ an instrument that measures charge*uantitati"ely is called an electrometer .

Pith ball electroscope from the &01(s! shoing attraction to charged obect

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-o it or2s

Pith-ball electroscope

The pith)ball electroscope! in"ented by #ritish schoolmaster and physicist 3ohn Canton in &145!

consists of a small ball of some lighteight nonconducti"e substance! originally a spongy plantmaterial called pith! although modern electroscopes use plastic balls. The ball is suspended by a

sil2  thread from the hoo2 of an insulated stand. In order to test the presence of a charge on an

obect! the obect is brought near to the uncharged pith ball. If the obect is charged! the ball ill be attracted to it and mo"e toard it.

The attraction occurs because of induced polari+ation 

of the atoms inside the pith ball. The pith

is a nonconductor ! so the electrons in the ball are bound to atoms of the pith and are not free to

lea"e the atoms and mo"e about in the ball! but they can mo"e a little ithin the atoms. See

diagram at right. If! for e6ample! a positi"ely charged obect (B) is brought near the pith ball (A)!the negati"e electrons (blue) in each atom (yellow ovals) ill be attracted and mo"e slightly

toard the side of the atom nearer the obect. The positi"ely charged nuclei (red) ill be repelledand ill mo"e slightly aay. Since the negati"e charges in the pith ball are no nearer the obect

than the positi"e charges (C)! their attraction is greater than the repulsion of the positi"e charges!

resulting in a net attracti"e force. This separation of charge is microscopic! but since there are so

many atoms! the tiny forces add up to a large enough force to mo"e a light pith ball.

The pith ball can be charged by touching it to a charged obect! so some of the charges on the

surface of the charged obect mo"e to the surface of the ball. Then the ball can be used to

distinguish the polarity of charge on other obects because it ill be repelled by obects charged

ith the same polarity or sign it has! but attracted to charges of the opposite polarity.

Often the electroscope ill ha"e a pair of suspended pith balls. This allos one to tell at a glance

hether the pith balls are charged. If one of the pith balls is touched to a charged obect! charging

it! the second one ill be attracted and touch it! communicating some of the charge to the surfaceof the second ball. 7o both balls ha"e the same polarity charge! so they repel each other. They

hang in an in"erted 8,8 shape ith the balls spread apart. The distance beteen the balls ill gi"e

a rough idea of the magnitude of the charge.

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Gold-leaf electroscope

%old leaf electroscope shoing electrostatic induction

The gold)leaf electroscope as de"eloped in &101 by #ritish clergyman and physicist Abraham#ennet! as a more sensiti"e instrument than pith ball or  stra blade electroscopes then in use. It

consists of a "ertical metal rod! usually  brass! from the end of hich hang to parallel strips of

thin fle6ible gold leaf . A dis2 or ball terminal is attached to the top of the rod! here the chargeto be tested is applied.To protect the gold lea"es from drafts of air they are enclosed in a glass

 bottle! usually open at the bottom and mounted o"er a conducti"e  base. Often there are grounded 

metal plates or foil strips in the bottle flan2ing the gold lea"es on either side. These are a safetymeasure/ if an e6cessi"e charge is applied to the delicate gold lea"es! they ill touch the

grounding plates and discharge before tearing. They also capture charge lea2ing through the air

that could accumulate on the glass alls! and increase the sensiti"ity of the instrument. In precision instruments the inside of the bottle as occasionally e"acuated! to pre"ent the charge

on the terminal from lea2ing off through ioni+ation of the air.

$hen the metal terminal is touched ith a charged obect! the gold lea"es spread apart in a 8,8.

This is because some of the charge on the obect is conducted through the terminal and metal rodto the lea"es. Since they recei"e the same sign charge they repel each other and thus di"erge. If

the terminal is grounded by touching it ith a finger ! the charge is transferred through the human

 body into the earth and the gold lea"es close together.

The electroscope can also be charged ithout touching it to a charged obect! by electrostaticinduction. If a charged obect is brought near the electroscope terminal! the lea"es also di"erge!

 because the electric field of the obect causes the charges in the electroscope rod to separate.

Charges of the opposite polarity to the charged obect are attracted to the terminal! hile chargesith the same polarity are repelled to the lea"es! causing them to spread. If the electroscope

terminal is grounded hile the charged obect is nearby! by touching it momentarily ith a

finger! the same polarity charges in the lea"es drain aay to ground! lea"ing the electroscopeith a net charge of opposite polarity to the obect. The lea"es close because the charge is all

concentrated at the terminal end. $hen the charged obect is mo"ed aay! the charge at the

terminal spreads into the lea"es! causing them to spread apart again.

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Gold-leaf electroscopes

9Condensing9 electroscope! Rome :ni"ersity physics dept.

Electroscope from about &;&( ith grounding electrodes

 inside ar! as described abo"e

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<olbe electrometer! precision form of gold)leaf instrument. This has a light pi"otedaluminum "ane hanging ne6t to a "ertical metal plate. $hen charged the "ane is repelled by

the plate and hangs at an angle.

-omemade electroscope! &;((

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Charging an Electroscope by Induction Using a Negatively

Charged Balloon

An electroscope is a common demonstration apparatus used by physics teachers to illustrate

electrostatic principles of charging and charge interactions. The electroscope is most commonlyused as a charge)detecting de"ice. The electroscope shon in the animation belo consists of a

 plate =near the top>! a support stand =hich connects to the plate and e6tends through the centerof the scope>! and a needle hich rests upon the support stand and is free to rotate about its pi"ot.

The plate! support stand! and needle are all made of a conducting material hich allos for both

the free flo of electrons and the distribution of any e6cess charge throughout the electroscope.#y obser"ing any deflection of the needle! the presence of charge in either the electroscope or a

nearby obect can be determined.

One common demonstration performed ith the electroscope in"ol"es the induction process of

charging. In the induction process of charging! a charged obect is brought near to but not

touching the electroscope. The presence of the charged obect abo"e the plate of the electroscope!induces electrons ithin the electroscope to mo"e accordingly. $ith the charged obect still held

abo"e the plate! the electroscope is touched. At this point electrons ill flo beteen theelectroscope and the ground ! gi"ing the electroscope an o"erall charge. $hen the charged obect

is pulled aay! the needle of the electroscope deflects! thus indicating an o"erall charge on the

electroscope. The process of charging an electroscope by induction using a negati"ely charged balloon is depicted in the animation belo.

As shon in the animation abo"e! the presence of the negati"ely charged balloon abo"e the plateof the electroscope ill induce the mo"ement of electrons from the plate of the electroscope to

the support and needle of the electroscope. This is e6plained by the like charges repel  principle.

The negati"ely charged balloon repels the negati"ely charged electrons! thus forcing them to

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mo"e donards. Once the electrons lea"e the plate and enter the needle! both plate and

support?needle ac*uire an imbalance of charge. The plate ac*uires an e6cess of positi"e charge

=since electrons ha"e left this once neutral region> and the support?needle ac*uires an e6cess ofnegati"e charge =since electrons ha"e entered this once neutral region>.

Once charge ithin the electroscope has been polarized  =i.e.! separated into opposite types>! the bottom of the electroscope is touched by a finger. #eing repelled by the negati"ely charged

 balloon! electrons from the electroscope e6it and enter into the ground. Once more! this processis dri"en by the principle that like charges repel . The electrons! ha"ing a mutual repulsion for

one another and for the negati"ely charged balloon! choose to e6it the electroscope and enter into

the larger region. #y doing so! the electrons are able to distance themsel"es and so minimi+e therepulsi"e interactions. It is at this point in the induction process that the electroscope ac*uires an

o"erall charge. Since electrons ha"e left the electroscope! the o"erall charge on it is positi"e. In

general! the induction process ill alays place a charge on the obect hich is the opposite typeof charge possessed by the obect used to charge it.

It might be noted in the animation abo"e that hile the departure of electrons may lea"e theelectroscope ith an o"erall charge! the needle is still not deflected. The e6cess of positi"e

charge remains locali+ed in the plate of the electroscope! being strongly attracted to the negati"echarge of the balloon. Once the balloon is ithdran! there is a mo"ement of any remaining

electrons in such a ay that the positi"e e6cess charge becomes distributed about the entire

electroscope. At this point! the needle of the electroscope becomes deflected and shos the presence of charge.