Static Electricity 2000 Edition

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STATIC ELECTRICITY
2000 Edition
is the force that one experiences or measures around a
charged object.
5.4.2
Field meters are calibrated to measure the electric field
in units of volts per unit distance, typically in kilovolts per
meter. In most cases, the measurements are proportional to
the net static electric charge on the object being measured.
Field meters are called field mills or charge locators, depending
on their principle of operation and level of sophistication.
Since the charge density on or in a nonconductor is typically
not uniform, measurements should be taken at several loca-
tions.
5.4.3
Charged nonconductors exist in many forms, such as
sheets, films, webs, powders, liquids, process rolls, and extru-
sions. Charges on these materials and objects will produce
electric fields that will be influenced by the instrument, by the
observer, and by other nearby conductive, semiconductive, or
insulating materials. As a result, the electric field measured by
the instrument will generally be different from the electric
field present before the instrument was introduced. This phe-
nomenon is a result of a change in capacitance.
5.4.4
The forces between electric charges exhibit themselves
at a distance. For this reason, the effects of accumulated
charge can be observed as the charged objects are
approached. For example, the hands and arms of a person
who approaches a highly charged object will tingle and might
even draw a spark as the surface of the skin and the hair
become “charged.” Sometimes these observations and sparks
provide the first indication that a potentially hazardous condi-
tion exists. This charging of the human body can happen even
if the person is well grounded.
5.5 General Practices.
5.5.1
The primary instrument for locating a charge on either
a conductor or a nonconductor is the noncontact voltmeter or
field meter. As its name implies, the instrument does not con-
tact the charged surface directly. Rather, it senses the magni-
tude and the polarity of the electric field that exists at its
sensing aperture. As stated in 5.4.3, the instrument and its sens-
ing aperture disturb the electric field around the charge to be
measured, so the meter reading does not accurately indicate
the actual magnitude of the field when the meter is not
present. Aside from this deficiency, the field meter is an inex-
pensive and valuable tool for locating a static electric charge.
5.5.2
In some cases, primarily cases involving flowing liquids
and flowing bulk solids, it is easier to collect a sample of the
charged material in an isolated vessel or cup, referred to as a
Faraday cup, and to use an electrometer to measure the stream-
ing current or net charge flowing to the cup receiving the
charged material.
5.5.3
The proper use of instruments to evaluate the magni-
tude of charge accumulation in specific applications is further
discussed in Chapters 7 through 9.