measurements of atmospheric electric field, conductivity, dry and wet-bulb temperatures and wind speed and direction are also made. The data have been used to compare the
values of space charge density obtained by the two techniques under different meteoro- logical conditions.
2. Instrumentation
A cubical Faraday cage of 3.6 m side is erected at the Atmospheric Electricity Ž
X X
. Observatory at Pune
18832 N, 73851 E, 559 m above sea level , India, using a galvanised iron wire mesh having 5 meshes per centimeter and made of 0.5 mm thick
Ž .
Ž .
Ž .
wire and angle iron Fig. 1 . Earlier, Vonnegut and Moore 1958 and Anderson 1966 have used such a wire mesh to construct Faraday cages for their measurements. Any loss
of ions entering the cage by attachment at the screen will be discussed in the next section. The cage has a small screened access door on one of its side. All sides of the
cage are interconnected and earthed. A small a.c. field-mill using a 92 W motor operating at 4000 rpm is fabricated and suspended in the center of the cage with a 3.2
cm diameter iron pipe through the top of the cage. The field-mill measures the potential
Ž created at the center of the cage by the space charge inside the cage Vonnegut and
Fig. 1. The Faraday cage at Atmospheric Electricity Observatory, Pune.
. Moore, 1958 . The mechanical design and the electronics used to process the signal of
field-mill is carefully designed to achieve a sensitivity of 0.2 Vrm and is found to be stable even after a continuous operation of several hours. Various factors, such as a large
2
. effective aera 201 cm
of the sensing stators, using a high speed motor to drive the rotors, grounding the shaft of the motor through a carbon brush, using the electronic
components with low temperature drift and sharply tuned amplifiers, combine to give the desired sensitivity, resolution and stability of the field-mill. The field-mill can sense
a field of 0.2 Vrm and has low noise-to-signal ratio. The zero-shift is observed to be less than 0.3 Vrm even after continuous operation of several hours. Potential, V , at the
c
Ž center of a cubical cage is related to space charge density by Vonnegut and Moore,
. 1958; Vonnegut et al., 1961
N s 780V rl
2
1
Ž .
c
where N is the space charge density in elementary chargesrcm
3
and l is the length of the side of cage in meters. Therefore, a potential of 0.2 V at the center of the cage
3
Ž
3
. corresponds to 12 elementary chargesrcm
1.92 pCrm . To ensure that the wire-mesh used to construct Faraday cage is fine enough to
adequately shield the volume of the air inside the cage from the outside electric fields, the variations in electric field observed inside the cage are compared to those observed
outside the cage measured with another field-mill kept flush with the ground and located 20 m away from the cage. These observations show that the two fields are not directly
correlated, thus confirming that Faraday cage provides adequate shielding to the field mill suspended inside it from the outside electric field and that it measures the potential
created only by the space charge inside the cage.
Ž .
Two identical filter apparatuses similar to that of Moore et al. 1961 , each enclosing a 10 = 10 = 15 cm absolute filter and a steelwool pre-filter in a shielding but insulated
galvanised iron tube, as shown in Fig. 2, are fabricated. An absolute filter is a glass-wool filter with fibers ranging from 0.5 to 3 mm in diameter. All-glass filter
Fig. 2. Schematic diagram of the filter apparatus.
medium is formed into closely pleated package with aluminium separators in a metallic frame. The filter used in our apparatus has a collection efficiency of greater than 99.9
for 3 mm particles. Each tube has a Teflon cone that insulates the electrostatic shield from the filter and prefilter assembly and extends to about 1 cm at its intake. Each tube
Ž .
also has a separate high-impedance amplifier AD 311K fitted outside the tube. The air intake constructed of Teflon avoids generation of point discharge under conditions of
Ž .
high electric field Moore et al., 1961 . It also serves the purpose of reducing the area of intake and thus reduces the non-uniformity of the rate of suction of air due to gustiness
Ž .
of the wind Bent, 1961 . Signal from the filter is fed to the amplifier. Air through each filter apparatus is drawn at a rate of 2 lrs with a separate blower. Flow-rate is measured
with a rotameter. The combination of the use of Teflon ring at the intake, providing more effective shielding of the filter from outside electrical effects by making the tube
inlet narrow, using a carbon coated Teflon coaxial cable for input connections, using the minimum length of this cable by mounting the amplifier on the tube itself, using the
Ž .
operational amplifiers with low temperature drift 10 mVr8C helps to achieve the desired accuracy and stability of the apparatus. Each apparatus is found to sense a space
3
Ž
3
. charge density of 16 elementary chargesrcm
2.56 pCrm and is found to have low
noise-to-signal ratio. The zero-shift is found to be less than 4 pCrm
3
even after several hours of continuous operation.
Fig. 3. Diurnal variations of electric field wind speed and space charge density measured by two filter apparatuses placed at 0.1 and 1 m above ground and a Faraday cage on January 14, 1996.
To check the identity of the two filter apparatus, both are first kept at 1-m height with their intakes close to each other. The flowrate through each apparatus is adjusted to be
equal to 2 lrs and the gains of the amplifiers in both apparatus are adjusted to be the same. The outputs of the two apparatus are observed to be within 4 of each other with
such arrangements.
The two filter apparatuses are placed — one at 0.1 m and other at 1 m above the ground — at a distance of 14 m from the Faraday cage. Outputs of both filter
apparatuses and the field-mill are fed to a data-logger which recorded 1-min average values. All the three instruments are run continuously for several weeks. Zero-shifts in
all three sets of apparatus are checked by switching off the power to the blowerrmotor and is corrected at least thrice daily. As the time at which the zero-shift first appears is
not exactly known, the data for the periods when zero-shift in any apparatus is found to be appreciable, are not considered.
Simultaneous measurements are also made of the atmospheric electric field with an a.c. field-mill kept flush with the ground in a pit 20 m away from Faraday cage and the
electric conductivity of both polarities with two Gerdien’s condensers kept 1 m above the ground close to the two filter apparatus. Outputs of these apparatuses are also fed to
the same data-logger.
Atmospheric temperatures and wind speed and direction are recorded with a weather monitor installed 50 m away from the Faraday cage.
Fig. 4. As in Fig. 2 on February 4, 1996.
3. Results
