• Understand how electroscope is related to Coulomb Force Law
  
• Electroscope and Electrophorus
  
• Charged rods brought near the electroscope: demonstration
  
• Electroscope and versorium
  
• Gold leaf electroscope class experiment
  
• Build an Electroscope
  
• Build Your Own Electroscope and Radiation Detector
  
• Use Everyday Materials to Detect Microscopic Particles
  
• Build your own electroscope
  

See also Electrometer

An electroscope is a scientific instrument that is used to detect the presence and magnitude of electric charge on a body.

There are two classical types of electroscopes:

• Pith-ball electroscope; and
• Gold-leaf electroscope.

Credit: thesciencefair.com

Pith-ball electroscope

The pith-ball electroscope was invented by John Canton. Basically, a light pith ball is suspended by a silk thread from the hook of an insulated stand. In order to test the presence and magnitude of a charge on an object, the object is brought near to the pith ball without coming into physical contact with the object.^[1]

If the pith ball is attracted to the object, the inference is that the object's charge is of opposite polarity or sign to that of the pith ball, while if the pith ball is repelled, the two objects have charge of the same sign, though not necessarily the same magnitude.

Two balls are commonly used, and become separated when charged.

Gold-leaf electroscope

The gold-leaf electroscope was developed at the end of the eighteenth century, by Abraham Bennet, as a more sensitive instrument than similar electroscopes using pith balls or straw blades then in use.^[2] It comprises an insulated metal rod, usually brass, at one end of which are fitted two pieces of thin gold leaf and with a metal disc at the other end where the charge to be tested is applied.^[2] To protect the gold leaves from drafts of air they are enclosed in a glass bottle open at the bottom, mounted over a conductive base. Usually there are grounded metal plates or rods at the sides of the foil sections, to impede excessive divergence, capture charge leaking trough the air that could accumulate in the glass walls, and increase the sensitivity of the instrument. As of 2007, aluminum foil is also used instead of gold.

When the metal disc is touched with a conductive charged object the leaves of the electroscope diverge. This is because the charge on the object is conducted through the disc and metal rod to the leaves.^[2] As they receive the same sign charge they repel each other and thus diverge. If the metal rod is touched with a finger, the charge is transferred through the human body into the earth and the leaves of electroscope close together.

If the electroscope is brought near a charged object, without touching it, the leaves also diverge, because charges in the disk with identical polarity to the charged object are repelled to the leaves. If the electroscope is touched, and hence grounded, in this condition, the leaves close, but spread again if the charged object is moved away. The electroscope becomes charged with polarity opposite to the polarity of the charge in the object. The electroscope was charged by electrostatic induction. The electroscope can be used to detect electric current.

It should be noted that this type of electroscope usually acts as an indicator and not a measuring device, although it can be calibrated.

Gold-leaf electroscope

Experiments with an electroscope

1. Touch the terminal with a charged conductor. Electrons flow into or out of the electroscope until its potential equals that of the conductor. The net surplus or deficit of electrons on the leaves causes them to repel each other. The charge that transfers, and therefore the deflection, are functions of the original potential difference between the conductor and the uncharged electroscope.
2. Place an isolated, charged conductor in contact with the terminal. The proportion of the conductor's charge that ends up on the terminal depends on their relative capacitances. This charge remains on the electroscope when you take the conductor away. The electroscope indicates this charge.
3. Bring a charged insulator near to the terminal but don't allow any charge to transfer between them. In this case, the deflection indicates the net charge on the electrons that have been temporarily pushed towards or away from the leaves. This charge is related to the charge density on the part of the insulator near to the terminal, and the relative shapes and positions of the two. What the electroscope is measuring in this case is quite vague, because there is no sharp boundary between the electron-rich and electron-depleted regions of the electroscope.
4. Place a charged insulator on or near the terminal, ground the terminal and then withdraw the insulator. This leaves the electroscope with a net charge. The electrons redistribute themselves over the terminal and the leaves, causing the leaves to repel each other. The deflection is monotonically related to the net charge left behind on the electroscope, in a similar way to the previous case.

References

Bibliography

• [Anon.] (2001) "Electroscope", Encyclopaedia Britannica
• Elliott, P. (1999). "Abraham Bennet F.R.S. (1749-1799): a provincial electrician in eighteenth-century England" (PDF). Notes and Records of the Royal Society of London 53(1): 59-78.  (

See also

• Electrometer

External links

• Pith-ball electroscope. Physics demonstration resource. St. Mary's University. Retrieved on 2007-09-02.