Quadrant
Electrometers |
Paul Frame, Oak Ridge Associated Universities |
|
The
primary function of an electrometer is to measure an electrical potential
(volts) or charge (coulombs) -
charge and electric potential are directly related. By measuring
the change in the accumulated charge over time, the current (amperes) can
also be determined. One ampere
is equivalent to one coulomb per second.
The basic quadrant
electrometer was developed by Lord Kelvin (William Thomson) in the 1860s.
The earliest versions were usually housed inside a “bird cage” or box.
The bird cage electrometers got their name from the Faraday cage that was
used to protect the instrument from stray electrostatic charges. In some
cases, a glass bell jar protected them from the air currents which could
affect their operation. The other common approach was to house the
electometer inside a wooden box, the front of which was made of
glass.
However, it wasn’t
until the development of the Dolezalek quadrant electrometer in 1896 that
a device was available with the sensitivity needed to measure the very
small currents (picoamps) associated with the ionization chambers used to
measure radioactive samples.
Among other things, the
Dolezalek electrometer and its later derivations protected the quadrants
inside a cylindrical metal housing that had one or possibly two small
glass windows.
Quadrant electrometers could be operated in a number
of ways but the most common method was to measure the rate of deflection of the electrometer’s vane
(aka needle)
suspended by a fine wire or fiber.
|
The potential being measured caused a
butterfly-shaped vane ( ca. 1 – 3” across) to turn. The magnitude of
this deflection was related to the potential (or charge). The rate of the
deflection was related to the current. The early electrometers used vanes
that were solid metal and relatively heavy.
In contrast, the vane of the Dolezalek electrometer was very thin
and light -
it was sometimes made of aluminum, sometimes of metal-coated paper.
One end of a short rigid wire was connected to the middle of the vane
while the other end of the wire had a small hook.
The backside of a tiny mirror (a few mm in diameter) was attached
roughly half way up the wire. The hook was connected to the end of a thin
flexible wire or fiber so that the vane and mirror were suspended in air
and free to rotate.
|
 |
 |
The vane itself was inside, but not in
physical contact with, a metal “pill box” shaped device consisting of
four quadrants. In the photo on the left, the quadrants have been opened up so that the vane is visible.
Each quadrant was electrically connected
to the quadrant diagonally opposite it so that they had the same charge.
One pair of quadrants had a positive charge and the other pair had
a negative charge. The
electrical charge on the vane caused it to take up a particular
orientation within the quadrants. If the potential difference between the
vane and the quadrants changed (e.g., due to current from the ion
chamber), the vane and the mirror would rotate.
|
Prior to the
measurement, the system was adjusted (mechanically and electrically) so
that the vane was positioned inside the quadrants as shown in the
accompanying figure. Half of
each lobe of the vane was inside one quadrant while the other half was inside the
adjacent quadrant.
To determine the position of the vane, a
beam of light was shone through the window in the electrometer case so
that it reflected off the mirror onto a scale usually positioned one meter
away. As the current from the
ion chamber changed the potential difference between the vane and the
quadrants, the vane rotated and the reflected beam of light moved across
the scale. The time requited
to move across a specified number of divisions on the scale could be
related to the activity of the sample by calibration with a known source.
In some cases, a calibration wasn’t necessary, only the relative rates
of deflection under different experimental conditions might have been of
interest.
|
 |
Early Quadrant
Electrometers
Note that the quadrant electrometers of the 1800s did not provide
the sensitivity or the reliability that was required for radioactive work.
|
The early bird cage
and box-like quadrant electrometers maintained the vane at a constant
potential while the potential/charge being measured was applied to one
pair of quadrants. The other pair of quadrants was grounded. The mechanism
employed to maintain the potential on the vane was somewhat cumbersome: a
wire was suspended beneath the vane so that the free end at the bottom
(usually attached to some sort of weight) was immersed in a sulfuric acid
solution. The sulfuric acid was contained inside a Leyden jar to which a
charge had been applied. A Leyden jar, actually an early form of
capacitor, is simply a glass jar, the bottom portion of which is lined on
the inside and outside with metal foil. The sulfuric acid provided the
electrical connection between the foil on the inside of the jar and the
wire connected to the electrometer vane.
|
 |
Dolezalek
Electrometer.
The Dolezalek
electrometer, invented by the Hungarian, Friedrich Dolezalek (1873 -
1920), represented a significant improvement over earlier versions of
quadrant electrometers by virtue of its increased sensitivity. It was
invented in the same year that radioactivity was discovered (1896) and it
quickly became a favorite of those investigating radioactive substances
(e.g., Ernest Rutherford). Dolezalek
spent most of his career in Germany and his research spanned a variety of
fields: physics, chemistry, and electrical engineering.
|
The Dolezalek quadrant
electrometer differs from previous designs in several respects:
1. The vane is lighter. It was
usually made of paper coated with a thin layer of metal (e.g., silver)
although thin aluminum was sometimes used. Older vanes were solid metal.
2. The quadrants are smaller.
3. The mirror and vane were
suspended by a metal coated quartz fiber rather than a phosphor-bronze
strip.
4. The
Leyden jar beneath the quadrants has been eliminated.
|
 |
The following diagram shows how a Dolezalek
electrometer and an ionization chamber might have been configured to
measure the activity of a radioactive substance. In this example a fixed potential
(e.g., 100 - 200 volts) is maintained between one pair of quadrants
(grounded) and the vane. The charge collected by the ionization chamber
accumulated on the other pair of quadrants. |
 |
Major
Types of Twentieth Century Quadrant Electrometers
1.
Dolezalek
2.
Compton - a variant of the Dolezalek electrometer. The major
difference being that the Compton electrometer permitted a mechanical
adjustment of the position of one of the quadrants.
3.
Hoffman - the most sensitive of the quadrant electrometers.
Actually, it was a binant (rather than quadrant) electrometer that used a vane with a single
lobe. It employed heat sinks to reduce thermally generated air currents
that could affect the vane and it was partially evacuated so that it
operated at reduced pressure (a few millimeters of mercury).
|
|
Electrometer
Museum
Directory |