Basic Electrostatics System
012–07227A
8
NOTE:
The proof planes can be used to test
for charge polarity on conductors of any shape.
However, if you want accurate readings of
charge density, the conductor surface sampled
has to be considerably larger than the disk of
the proof plane and have a relatively large
radius of curvature at the point of contact.
Variable Capacitor (ES-9079)
The PASCO experimental variable capacitor
consists of two metal plates, 20 cm in
diameter, which can be adjusted to various
separations. The movable plate is mounted
on a calibrated slide which gives the plate
separation directly in centimeters. Binding posts
are provided for electrical connection to each plate.
Three plastic spacers are attached to the fixed plate
so that when the movable plate is made to touch
these spacers, the plate separation is 1 mm.
Use a low-capacitance cable to connect the plates
to the electrometer. Keep the leads of the cables
separated as much as possible to minimize
capacitance.
It is extremely important that the plates of the
capacitor remain parallel. It is possible that
through mishandling, they will cease to be
parallel and adjustment must be made. On the back
side of the fixed plate is a second, smaller plate
with three set screws. By adjusting these three
screws, the two plates may be made parallel.
Keep the clear acrylic plate supports clean to prevent
charge leakage from the plates.
Figure 10. Proper use of a Proof Plane to Sample Charge.
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Figure 11. Basic Variable Capacitor.
charged conductive
sphere
surface of the
proof plane
IS
NOT tangent to
the surface of the
conductor
surface of the
proof plane
IS
tangent to the
surface of the
conductor
NO!
YES!