Electroscope

Pith-ball electroscope

In 1731, Stephen Gray used a simple hanging thread, which would be attracted to any nearby charged object. This was the first improvement on Gilbert's versorium from 1600.

The pith-ball electroscope, invented by British schoolmaster and physicist John Canton in 1754, consists of one or two small balls of a lightweight nonconductive substance, originally a spongy plant material called pith,{{cite book | last = Derry | first = Thomas K. |author2=Williams, Trevor

The attraction occurs because of induced polarization{{cite book |url-status = dead |archive-url = https://web.archive.org/web/20140131021101/http://books.google.com/books?id=pPleL5NOtb4C&pg=PA329&dq=%22pith+ball+electroscope%22+induction&hl=en&sa=X&ei=UOL_Tr-0GeeRiQK76aTCCg&ved=0CEcQ6AEwAQ#v=onepage&q=%22pith%20ball%20electroscope%22%20induction&f=false |archive-date = 2014-01-31 | access-date = 2012-01-01}}{{cite book

If the external object (B) instead has a negative charge, the positive nuclei of each atom will be attracted toward it while the electrons will be repelled away from it. Again, this causes opposite charges to be closer to the external object than charges of the same polarity, resulting in a net attractive force.

The pith ball can be charged by touching it to a charged object, so some of the charges on the surface of the charged object move to the surface of the ball. Then the ball can be used to distinguish the polarity of charge on other objects because it will be repelled by objects charged with the same polarity or sign it has, but attracted to charges of the opposite polarity.

Often the electroscope will have a pair of suspended pith balls. This allows one to tell at a glance whether the pith balls are charged. If one of the pith balls is touched to a charged object, charging it, the second one will be attracted and touch it, communicating some of the charge to the surface of the second ball. Now both balls have the same polarity charge, so they repel each other. They hang in an inverted 'V' shape with the balls spread apart. The distance between the balls will give a rough idea of the magnitude of the charge.

Gold-leaf electroscope

The gold-leaf electroscope was developed in 1787 by British clergyman and physicist Abraham Bennet, as a more sensitive instrument than pith ball or straw blade electroscopes then in use. It consists of a vertical metal rod, usually brass, from the end of which hang two parallel strips of thin flexible gold leaf. A disk or ball terminal is attached to the top of the rod, where the charge to be tested is applied. To protect the gold leaves from drafts of air they are enclosed in a glass bottle, usually open at the bottom and mounted over a conductive base. Often there are grounded metal plates or foil strips in the bottle flanking the gold leaves on either side. These are a safety measure; if an excessive charge is applied to the delicate gold leaves, they will touch the grounding plates and discharge before tearing. They also capture charge leaking through the air that accumulates on the glass walls, increasing the sensitivity of the instrument. In the precision instruments the inside of the bottle was occasionally evacuated, to prevent the charge on the terminal from leaking off through the ionization of the air.

When the metal terminal is touched with a charged object, the gold leaves spread apart in an inverted 'V'. This is because some of the charge from the object is conducted through the terminal and metal rod to the leaves. Since the leaves receive the same sign charge they repel each other and thus diverge. If the terminal is grounded by touching it with a finger, the charge is transferred through the human body into the earth and the gold leaves close together.

The electroscope leaves can also be charged without touching a charged object to the terminal, by electrostatic induction. As the charged object is brought near the electroscope terminal, the leaves spread apart, because the electric field from the object induces a charge in the conductive electroscope rod and leaves, and the charged leaves repel each other. The opposite-sign charge is attracted to the nearby object and collects on the terminal disk, while the same-sign charge is repelled from the object and collects on the leaves (but only as much as left the terminal), so the leaves repel each other. If the electroscope is grounded while the charged object is nearby, by touching it momentarily with a finger, the repelled same-sign charges travel through the contact to ground, leaving the electroscope with a net charge having the opposite sign as the object. The leaves initially hang down free because the net charge is concentrated at the terminal end. When the charged object is moved away, the charge at the terminal spreads into the leaves, causing them to spread apart again.

Bohnenberger electroscopes were widely used in 19th-century experimental physics and appear in university laboratories, teaching collections, and scientific manuals throughout Europe. The design influenced later high-sensitivity electroscopic instruments.

Eberbach & Son electroscope instruments were designed primarily for educational and laboratory use, following classical electroscope principles while emphasizing robustness and standardized construction for teaching environments.

While they did not introduce new electroscopic principles, they played a role in the standardization of electrostatics instruciton in North America. File:Gold-leaf electroscope-2.jpg|Condensing electroscope, Rome University physics dept. File:Gold leaf electroscope with ground strips.png|Electroscope from about 1910 with grounding electrodes inside jar, as described above File:Gold leaf electroscope homemade.jpg|Homemade electroscope, 1900

Footnotes

References

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