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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Electroscope | 2/2 | https://en.wikipedia.org/wiki/Electroscope | reference | science, encyclopedia | 2026-05-05T09:42:35.181897+00:00 | kb-cron |
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.
The Bohnenberger electroscope was developed in the early 19th century by the German physicist Johann Gottlieb Friedrich von Bohnenberger as an improvement on earlier gold-leaf electroscopes. The instrument employed a single gold leaf suspended between two oppositely charge plates, increasing sensitivity and allowing clearer detection of both the presence and sign of an electric charge. 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 instruction in North America.
== See also ==
== Footnotes ==
== External links == "Pith-ball electroscope". Physics demonstration resource. St. Mary's University. Retrieved 2015-05-28. "Computer simulation of electroscopes". Molecular Workbench. Concord Consortium. Archived from the original on 2022-07-03. Retrieved 2008-02-03. "Pith Ball and Charged Rod Video". St. Mary's Physics YouTube Channel. St. Mary's Physics Online. Archived from the original on 2021-12-22.