Volta-Type Condensing Electroscope (ca. 1930s)
 

This is a gold leaf electroscope that was manufactured by Griffin & Tatlock Ltd. of London, England. Based on its general appearance, and the fact that the company name was first used in 1929, I would guess that it dates from the 1930s.

Like any gold-leaf electroscope, it was intended for qualitative demonstrations of various electrostatic phenomena.  

The body is glass. The wooden base is mahogany. The circular Volta plate and the vertical rod (the lead-in) from which the gold leaves would be suspended, are brass.  No insulator is necessary since the glass body is non-conductive.

Alas, the two gold leaves are missing.

The bottom of the glass jar is in contact with a tin foil disk attached to the wooden base. During use, this foil disk might or might not be grounded.

Also attached to the foil are two "earthing strips" that run half way up the glass wall of the jar.

Earthing strips (aka screening strips) like these, or brass rods like those in the Ducretet Gold Leaf electroscope, are commonly found in electroscopes that have a non-conductive glass body. They seemed to have had two functions. First, they shielded (screened) the leaves from static charges on the glass or nearby charged objects. For this to work, the foil disk to which the strips were connected had to be grounded.

Quoting Samuel S. Richardson:

"It is often necessary to shield delicate instruments from the disturbing effect of neighboring charged bodied. This may be done by placing them inside a metal hollow conductor. If a gold-leaf electroscope is placed inside a can it will be shielded from the influence of a charged rod outside. An electroscope with glass walls is screened by tinfoil strips or wire gauze attached to the inner surface of the glass. An electrometer is provided with a brass case, or "bird-cage", which serves the same purpose."

The other function attributed to these strips was that they make the electroscope more sensitive.

Quoting Arthur L. Kimball:

"There are induced charges on the metal strips inside the electroscope. These charges are opposite to the charge on the [gold] leaves and increase their divergence."

 

The needle-like object attached to the top of the disk creates a corona discharge point. Its purpose was to demonstrate the phenomenon of a corona discharge (i.e., the leakage of charge from a point). It has a threaded end and can be affixed to the plate so that it is either pointing up or down.

The drawing to the right (from Science Readers Book 7 by Vincent T. Murche, 1906) depicts an almost identical electroscope being charged with a rod. It was also possible to charge the electroscope using an electrophorus.  For an explanation of how an electroscope can be charged and how an electrophorus works, click here. 

Drawing

Size: ca. 10" high, 3" diameter

Reference

Kimball, A.L. A College Text-book of Physics. 1911.

Richardson, S. S. Magnetism and Electricity and the Principles of Electrical Measurement. 1908.

 
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Last updated: 07/25/07
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