electroscopes are very sensitive instruments useful in qualitative and
quantitative investigations of static electrification. They have been used for
many years for the accurate measurement of charge. Their operation is based on
the principle of like sign charge repulsion. Two sheets or leaves, cut longer
than they are wide and made of very thin, electrically conductive material, are
hung adjacent and virtually in contact with each other. The leaves are
ordinarily made of material sufficiently thin so that they have no rigidity and
hang down limply. When the leaves, which are electrically connected, become
electrically charged, they push apart from each other. The angle they form
correlates on the amount of electric charge on the leaves. See the figure
below. If the instrument is
shielded so that the capacitance is fixed, then the angle can be with some precision to
static voltage. It should be noted that the electroscope actually indicates potential,
not charge [Greenslade and Howe, 1981]. The leaf electroscope does not
distinguish positive from negative charge, though there are some simple methods for doing so.
In a calibrated electroscope, the two leaves are made of fine hammered gold leaf and the voltage is determined by viewing and measuring the separation angle with a low-power microscope. To meet the requirements of a robust apparatus for the lecture hall, the sensitivity of fragile gold leaf must be sacrificed in favor of more durable material. Good choices include (i) the conductive plastic film used to make antistatic bags (e.g., VelostatTM made by 3M Company) and (ii) aluminum-coated MylarTM film. VelostatTM is particularly easy to work with, sufficiently robust to be used in the lecture hall, and sufficiently conductive so that, unlike MylarTM, it does not become permanently electrified.
All the important features of operation of the classical electrophorus may be demonstrated with an electroscope accessory mounted to a piece of aluminum or steel rod of minimum diameter 1/16". The rod is shaped so that the electroscope leaves may be attached at one end which has been bent horizon. The other end is attached to the top or, better yet, inserted into a hole drilled into the top of the electrode of the electrophorus. To avoid corona discharge, the rod should be polished smooth, with no sharp edges or scratches.
The photo below shows an electrophorus
equipped with a simple electroscope, the leaves of which have been made of the
black plastic film used for packaging and shipping electrostatically sensitive
leaves respond by spreading out quite noticeably as the electrophorus is lifted
off the charged TeflonTM plate. One nice advantage of using the electroscope
accessory is that it seems to work reliably even on warm, humid days when the
electrophorus is unable to produce sufficient charge for strong capacitive
sparks. Performance of the electroscope can be improved if the very end of the
metal rod from which the leaves are suspended is covered by a small piece of
conductive antistatic packing foam. This foam, visible in the photograph,
provides resistive grading of the electric field, thus substantially reducing
corona emission from the end of the rod.
A similar electroscope accessory can be made by bending the end of the wire opposite the leaves into the form of a horizontal base upon which the instrument sits upright. This simple apparatus can then be used with many different electrostatics demonstrations, including the dissectible capacitor, where it readily reveals how the electric surface charge redistributes itself when the electrodes are separated from one another.
Combining the electroscope with a dissectible capacitor was first the idea of Volta [Greenslade and Howe, 1980]. The arrangement mades it possible to study the way charges redistribute themselves as electrical conductors are moved about. The photograph below shows the potential multiplication that results if the capacitor is first charged with a 9 volt battery, then disconnected from the battery, and raised up. The leaves of the electroscope reveal a significant increase in the electrostatic potential.
This photograph and the one just above it on this page are similar in virtually every respect, except for the means by which the movable electrodes gain their initial charge. For the electrophorus, the electrode is charged by the INDUCTION mechanism under the influence of the permanent charge imparted to the insulating TeflonTM plate when it is rubbed with a cloth. The electrode can be charged again and again because the charge on the insulating surface is "permanent.". On the other hand, the dissectible capacitor is charged by CONDUCTION using a voltage source. The source must be used to recharge the capacitor each time a spark is drawn.
Young scientists and hobbyists interested in electroscopes can check out the very nice web pages of San Fransisco's famous Exploratorium or
the Boston Museum of
Science for instructions on how to make simple, working devices out of
aluminum foil, paper clips, and other inexpensive materials. A very instructive animation showing the charging process for a somewhat different type of electroscope has been created. A good site to search for applications of the electroscope is in
the University of Maryland's Physics
Demonstration List. These pages also explain some experiments that can be
done with the electroscope. There is also a good library reference describing
construction of simple electroscopes [Ostlund and Dispezio,
T.B. Greenslade and R.H. Howe, "A modern use of Volta's electroscope," Physics Teacher, Vol. 19, No. 9, December, 1981 p. 614-615.
K.L. Ostlund and M.A. Dispezio, "Static electricity
dynamically explored," Science Scope, February, 1996, pp. 12-16.