012–07227A
Basic Electrostatics System
13
may observe that everywhere on the surface the charge has the same magnitude
and sign. This occurs for uniform charge distribution.
An important aspect of measuring charge distributions is charge conservation. The
proof plane removes some charge from the surface it samples. If the proof plane is
grounded after each measurement, the surface will be depleted of charge with
consecutive measurements. However, by not grounding the proof plane (and by not
letting it touch the ice pail), the charge on the surface is not depleted. That charge
which the proof plane removed for one measurement is always returned to the
surface when the next sampling is made.
NOTE: When the disk of the proof plane touches the surface
being sampled, it essentially becomes part of the surface. To
minimize distortion of the surface shape when sampling, hold
the proof plane flat against the surface, as indicated in the
accessory instructions. Please refer to the accessory instructions
for the details on how to use the proof planes.
Procedure:
1. Before starting, make sure the Faraday Ice Pail is properly grounded, with the
shield connected to earth ground. The electrometer, connected to the pail, must
also be grounded.
2. Place the two aluminum spheres at least 50 cm apart. Connect one of the
spheres to the Electrostatic Voltage Source (ES-9077) providing 1000 VDC. The
voltage source is to be grounded to the same earth ground as the shield and the
electrometer. The connected sphere will be used as a charging body.
3. Momentarily ground the other sphere to remove any residual charge from it.
4. Start the demonstration by sampling and recording the charge at several
different points on the sampling sphere. (The sphere that was grounded in step
2.) Choose points on all sides to represent an overall sample of the surface.
5. Now bring the 1000 VDC sphere close to the grounded sphere, until their
surfaces are about 1 cm apart. Turn the voltage source ON, then sample and
record the charge at the same points sampled before.
6. Momentarily ground the sampling sphere again, by touching one hand to the
grounded ice pail shield and the other hand to the sphere. (Make sure the ice
pail is grounded before doing this.) Again, sample and record the charge at the
same points sampled before.
7. Remove the 1000 VDC sphere until it is at least 50 cm away from the sampling
sphere. Again, sample and record the charge at the same points sampled before.