Kolbe's Electrometer/Electroscope by Max Kohl (ca. 1900)

   

What we have here is a box-like electroscope that was designed for classroom demonstrations of electrostatic phenomena. Like any electroscope, it could be used for the measurement of radioactive materials. The curved scale (mica) numbered 0 through 9 provides at least a limited capability for quantitative comparisons. If required, the scale could be calibrated in volts. Although the Kolbe electroscope was primarily used in a classroom or laboratory, it was rugged enough to be taken into the field.

This type of instrument was first described by Bruno Kolbe of St. Petersburg in 1889. Since Kolbe referred to it as an electrometer, that is what it is generally called. In reality (i.e., in my opinion), it is an electroscope. For a brief discussion about the difference between an electroscope and an electrometer, click here.

 
Kohl Kolbe 700  

A Kolbe electroscope employs a single leaf - the mangled fragment of a leaf can be seen attached to the vertical support rod in the above photo. The leaf was usually aluminum, but ours (or what is left of it) is gold. Paper leaves were also used. The metal components of the case were painted black in order to eliminate bothersome reflections of light. The four vertical sides of the case are glass while the top and bottom are sheet metal. The two glass sides that are not in view are covered on the inside with a metal screen to prevent static charges on the glass or nearby objects from affecting the movement of the leaf.  The brass knob on the left end of the wooden base is used to level the electroscope and the red connector on the right end is used to ground the case. The insulator that separates the vertical support rod from the metal top of the case appears to be quartz.

The maker's name is printed on the top of the case: Max Kohl A.G. Chemnitz. 

Kolbe electroscopes either had a hollow metal ball, or a horizontal disk, at the top of the vertical supporting rod. To minimize the loss of any stored electrical charge, these things had smooth rounded surfaces. Obviously, our electroscope has a disk. Such a disk was usually referred to as a condenser (or Volta plate) because it stored the charge applied to the electroscope. In general, a device called an electrophorus would be used to charge the condenser.

For an explanation of how an electroscope can be charged and how an electrophorus works, click here

For more information about the Kolbe electroscope/electrometer, refer to the description of the Leppin & Masche electroscope.

 
 

 

Notice that the numbers on the scale are upside down. The reason is that the electroscope was designed so that the movement of the leaf on the scale could be projected onto a large screen for better viewing. The projection would invert the image. How this might be done is shown in the figure to the right - the figure is from a 1908 text by Bruno Kolbe. The following quote is from that text:

"Of course, we must write the numbers the reverse way, so that they may appear right on the white screen."

 

kolbe

The image to the right is from a Max Kohl catalog (ca.1910-1920) that identifies this instrument as "Aluminum Electrometer (Kolbe's)." The catalog description follows:

"with projection scale for calibrating and amber tubes in ceramic plug, one ball 10 mm diameter, two condenser plates (lacquered) with one ebonite handle, one extra ebonite plug with amber tubes, conductor rod and paper leaves. The sheet iron house of the instrument is 130 mm high, 140 mm wide, 95 mm deep." Except for what you see in the photo, everything else listed in the catalog is missing. The price was 3 pounds. 

The following is the description of an identical instrument in a Baird & Tatlock catalog (1912):

"Electrometer (Kolbe's), for exact measurements, consisting of case with insulated rod and aluminum leaf. Two mica plates, one divided into degrees, are supplied with each apparatus." The price was 4 pounds.

 

 

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Clearly, it was a much better deal to buy it from Max Kohl than Baird & Tatlock.

Size: ca 9.5" tall. The chamber is 5.5" high 5.5" wide and 4" deep. The condenser (Volta) plate is ca. 3" in diameter. The wooden base is 9" x 4.5".

References

Kolbe, B. An Introduction to Electricity. Translated by J. Skellon. 1908.

Kolbe, B. Ein einfaches Elekrometer. Zeitschrift fur den Physikalischen und Chemischen Unterricht. April 1889;153-159.

Max Kohl A.G. Chemnitz. Physical Apparatus. Price List No. 50. Vols. II and III. No date.

I would like to express my thanks to Jean-François Loude for providing the above reference.

 

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Last updated: 07/25/07
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