Zeleny Electroscope
   The 1929 catalogue of the Central Scientific Company of Chicago lists this as "Apparatus for the Study of Radiation, Atomic and Molecular Structure", and priced it at $20.00 In the 1941 catalogue it has a less grandiose title, but the price remained the same, but by 1951 the price was up to $42.50.

   John Zeleny was a professor of physics at Yale University, and first described the apparatus in Phys. Rev.,  32,  581 (1911). The rate at which the electroscope leaf collapsed was proportional to the amount of ionization in the region between the grounded upper plate and the lower plate, attached to the gold leaf. 

   In use, a potential of about 100 V is applied between ground and the trapezoidal plate next to the gold leaf. The plate is moved sideways until it touches the leaf and transfers charge to it. The plate is then repelled, and only comes down when a conducting path to ground is provided by the ionized air. 

   A number of experiments could be done with the apparatus: ionization of air by alpha rays, ionization of air by beta rays, ionization by flames, a glowing splinter or a hot wire, the photoelectric effect and the range of alpha particles in air. 
   Attachments to the Zeleny electroscope could be used to show the effect of the grid in a triode vacuum tube. The heated filament is connected between the two pillars on the upper left-hand side of the apparatus. The electrons thus produced are attracted to a vertical plate (not shown) placed atop the round plate on top of the electroscope. Between the filament and the plate is the grid, composed of fine wires woven back and forth. As the grid is made increasingly negative, the current to the plate decreases, as in the case of a triode. 

   For the photoelectric effect demonstration, a vertical zinc plate is placed on top of the electroscope.

   This apparatus is at Kenyon College in Gambier, Ohio. I can vouch for the fact that it has not been used since 1964. 







   The Zeleny electroscope at the right is in the collection of  historical  scientific instruments at Case Western Reserve University in Cleveland, Ohio.

  This Zeleny Electroscope is on display at the University of Cincinnati physics department. The label on the accompanying box reads "Standard Scientific Company / Scientific Instruments / New York." A similar piece of apparatus is listed in the 1929 catalogue of the Chicago Apparatus Company at a price of $25.00. 

   The accompanying text notes that "The electroscope leaf is broad and moves in its own plane; the air resistance thus encountered is so small that it cuts through the air with a precise, rapid pendular motion. The leaf is automatically charged and recharged from a plate kept at about 100 volts. When an ionization current is being measured the leaf alternately approaches and "kicks" away from the plate, the frequency of these perpendicular motions being a measure of the current. ... The effect of any change in conditions is immediately noted by a change in the frequency of the oscillations. In making quantitative measurements, the time of a charged number of oscillations is taken, the "kick" of the leaf from the charging plate making a very definite point for taking time. The sensitivity of the electroscope can be varied over wide limits by simply moving the charging plate in and out." 
 

 
   This Zeleny Oscillating Electroscope in the Greenslade Collection is accompanied by an instruction manual published in 1919 by the Standard Scientific Company, 70 fifth Avenue, New York. The zinc electrode, used for experiments with the photoelectric effect, can be seen at the left-hand side of the picture, about half way up.

   The cost of the complete apparatus, including the wooden carrying case, was $60.00.
   This example of a Zeleny Electroscope was made by Prof. Joseph Naylor of DePauw University in Greencastle, Indiana. Naylor, who was a faculty member at Depauw from 1891 to 1925, was an expert at reproducing expensive apparatus using hardwood and brass as his raw materials. 
 
 

   Other electroscopes used for studies of radioactivity are listed under Radio Electroscopes. 
 
 

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