| Pieter van Musschenbroek, the professor
of natural philosophy at Leiden in Holland, was just about to get a nasty
surprise. When he touched the inner conductor of one of the world's first
condensers this day in 1745, he got a severe shock -- "I would not take a
second shock for the kingdom of France". The electric charge was supplied
by the electrostatic machine on the right, and the film of moisture on the
outside of the glass jar provided the outer conductor. The electric fluid,
was, of course, condensing in the water forming the inner conductor.
Jar itself is found on another page.
The apparatus on this page is often called the Condenser of Aepinus, after F.U.T. Aepinus (b. 1724). It appears that Ewald G. von Kleist (ca.1700-1748) independently made the discovery of the method of storing electrical charge a few months before van Musschenbroek.
REFERENCE: Thomas B. Greenslade, Jr., "The Discovery of the Leiden Jar",
Phys. Teach, 32, 536-537 (1994)
|This example is at the University of Texas in Austin. It appears to be the less-expensive version of the Kenyon College apparatus at the left, above. The 1888 catalogue of James. W. Queen of Philadelphia notes that it is the "Condenser of Epinus, outer plates each six and a half inches in diameter, in neat mahogany stand ... $16.00"||
| I found the adjustable capacitor at the left in the
demonstration apparatus collection at Cornell University. A similar one
was being used as part of an RC oscillator; the screw mechanism running through
it allows the spacing between the plates to be adjusted in small increments,
thus permitting the system to be tuned.
This apparatus is unmarked, but a very similar one is shown in the 1911 catalogue of W.G. Pye & Co. of Cambridge, England. It is listed as a "Sliding Condenser, with two movable plates 23 cms diam., accurately turned and well insulated, sliding on dove-tailed iron bed. Each plate is actuated by a screw for altering the distance between them... The maximum distance is 14 cms ... £14.10.0" (about $70)
This adjustable capaictor I found in the collection at Case Western Reserve University in Cleveland, Ohio. Unfortunately it is unmarked.
There are screw adjustments on the plates to render them parallel.
| Toward the end of the nineteenth century it became
possible to buy reliable fixed capacitors using oil as a dielectric. This
capacitor has the name of L.E. Knott of Boston engraved on it, and dates
from the first quarter of the twentieth century. It is marked "1/1, 1/10,
1/100. 1/1000", presumably microfarads.
It resides in the demonstration apparatus room at Cornell University.
"The distribution of electricity on the surface may
also be shown by means of the following apparatus: -- It consists of a
metal cylinder on insulated supports, on which is fixed a long strip of
tin-foil which can be rolled up by means of a small insulating handle.
A quadrant electrometer is fitted in metallic communication with the cylinder.
When the [tin-foil] is rolled up, a charge is imparted to the cylinder, by
which a certain divergence is produced. On unrolling the tin-foil this divergence
gradually diminishes, and increases at it is again rolled up. The quantity
of electricity remains the same, the electrical force, on each unit of sufrace,
is therefore less as the surface is greater."
In short, C depends on the area of the capacitor that
is charged with a constant charge Q. When C increases, V decreases.