A. Two-leaf Electroscopes         Here is the basic gold-leaf electroscope. Two thin gold leaves are suspended from the bottom of the brass top of the glass cylinder, and connected to the sphere at the top.     This apparatus is in regular use at Kenyon College in Gambier, Ohio, and is marked "James W. Queen". The 1888 Queen catalogue lists it as a "Plain gold Leaf Electrometer" and prices it at $3.75. Note that it is not an electrometer, but an electroscope; it is used to indicate the presence of charge, but not the potential associated with the charge.

The two electroscopes below are equipped with the bottom plate of Volta's condenser. The upper plate is of the same dimensions as the lower one, and has an insulating glass handle attached, and its lower surface is covered with varnish for insulation.     The condenser allows small quantities of charge to be detected. It is charged by touching the object under test to the lower plate, while grounding the top plate. If the lower plate has been positively charged, negative charge will be transferred to the upper plate. The ground connection is then broken, followed by disconnecting the test object. The capacity, C,  is then decreased by lifting up the upper place. Since the potential across the capacitor, V, is related to the constant charge Q by Q = CV, the potential increased and the electroscope leaves diverged.     REF: Thomas B. Greenslade and Richard H. Howe, "A Modern Use of Volta's Electroscope", The Physics Teacher, 19, 614-615 (1981)

The electroscope at the left is at Washington and Jefferson College in Washington, Pennsylvania.       At the right is an example in the Garland Collection of Classic Physics Apparatus at Vanderbilt University. The two gold leaves are missing, a common occurrence.     The upper condenser plate is missing in every Volta-type electroscope I have seen.

B. Bennet-Type Electroscopes
   The gold leaf electroscope invented in 1786 by Abraham Bennet (1750-1799) included two "earthing strips". Ganot notes that "The delicacy of this electroscope may be increased by adapting to the foot of the apparatus two metal rods, terminating in knobs; for these knobs, being excited by induction from the gold leaves, react upon them." This induction process is akin to neutral scraps of paper being attracted to an electrically-charged rod.

From the 1916 catalogue of L.E. Knott Apparatus Co., Boston: "Bennett's [sic] Gold Leaf Electroscope... This instrument is intended for sensitive work in testing electrification of a body and in determining the polarity of the same. In our design we use the best quality of electrical glass and further increase the conducting quality of this glass by grounding it. The jar used is of special mould which allows of the use of the various attachments without disturbing it. ... 8.75" The example at the left is at  Middlebury College in Vermont.    At the right is an instrument  from Bates College in Maine.

In this example, in the apparatus collection of Transylvania University in Lexington, Kentucky, Volta's condenser is attached to the side of the electroscope; the fixed condenser plate is missing. The other condenser plate is on a hinged rod, and is pivoted on the side to decrease the capacitance and increase the potential difference. As is often the case, the gold leaves are missing.

Most electroscopes are unmarked. However, the instrument at the far right, at Hampden-Sydney College in Virginia, is by Ducretet and Lejune of Paris. The 1880 catalogue of Ducretet et Cie lists this apparatus at 35 francs (about $7).     The apparatus at the near-right, from Allegheny College in Meadville, Pennsylvania, ought to be easy to identify because of its most unusual brass turnings. Indeed, they look just like the brass columns that appear again and again in apparatus made by Daniel Davis of Boston. The 1838 Davis catalogue, which is unfortunately not illustrated, lists a gold leaf electrometer at at $1.50. If the identification is correct, this is the only piece of Davis electrostatic apparatus that I have seen.     Soon afterward, Davis ceased to deal with electrostatic apparatus, and devoted his catalogue to apparatus for the study of magneto-electricity and electro-magnetism.

        Transylvania University                      University of Mississippi                  Glasgow University, Scotland

C. Electrometers or Electroscopes with Measuring Scales
 

The electroscope is uncalibrated and can only indicate the presence and relative magnitude of the charge on a conductor and its resulting electric potential. Electrometers, on the other hand, can be calibrated to read in Volts or kilo-Volts.     This example of Kolbe's aluminum Electrometer is listed at 55 Marks (about $13) in the catalogue that Max Kohl of Dresden, Germany, published ca. 1900. The light aluminum vane pivots and its position may be observed relative to a graduated scale on glass. This apparatus may be used with shadow projection so that a large audience can see the results of electrostatic experiments.        It is in the collection at the Physics Department at the University of Cincinnati.

   At the left below is a Max Kohl electrometer in regular use in lecture demonstrations at Kenyon College in Gambier, Ohio. This was imported by the Chicago Apparatus Company, the name of which appears on one of the legs. In the 1929 Chicago catalogue it was listed at $50.00. The calibration runs up to 10,000 V. The apparatus was developed by Prof. F. Braun of the University of Tübingen in Germany, and first described by him in an article in Wiedemann's Annalen in 1891. Again, this instrument is often used with shadow projection. At the right is a very similar piece of apparatus in the physics department of  Antioch University in Yellow Springs, Ohio.

This electroscope, by Richard Müller-Uri of Brunswick, Germany, was probably bought by Kenyon College in 1910.     The enclosed card gives the calibration as of "14 IX 10". The full-scale deflection of the leaf was 200 volts.     The leaf is held between two metal covers connected to the two side handles. These are slid outward for measurements. The insulation at the top is amber. The 1929 catalogue shows the device placed on an insulating stand, being used for measurements of atmospheric electricity.

At the left is a calibrated electrometer built by Prof. Joseph Naylor of Depauw University about 1900.

 

The electroscope at the left took me twenty years to identify. It is in the collection of Denison University in Granville, Ohio, and I first saw it in 1983 when cataloguing apparatus for a sesquicentennial exhibit of Denison's apparatus.      “Gold Leaf Electroscope, tilted rectangular pattern, with ebonite insulation to [the base] plate and amber insulation to the leaf, complete on stand with leveling screws, as designed by Mr. C.T.R. Wilson, F.R.S., Cavendish Laboratory, Cambridge … £2” (about $10). From the Catalogue of Scientific Apparatus Manufactured by W. G. Pye & Co., Cambridge, England. List No. 101, June 1911.