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Section 4: Basic measurements
Electrostatic interference is first recognizable when hand or body movements near the experiment
cause fluctuations in the reading. Pick-up from AC fields can also be detected by observing the
electrometer preamp output on an oscilloscope. Line frequency signals on the output are an
indication that electrostatic interference is present. Means of minimizing electrostatic interference
include:
Shielding. Possibilities include: a shielded room, a shielded booth, shielding the sensitive circuit,
and using shielded cable. The shield should always be connected to a solid connector that is
connected to signal low. If circuit low is floated above ground, observe safety precautions, and
avoid touching the shield. Meshed screen or loosely braided cable could be inadequate for high
impedances, or in string fields. Note, however, that shielding can increase capacitance in the
measuring circuit, possibly slowing down response time.
Reduction of electrostatic fields. Moving power lines or other sources away from the experiment
reduces the amount of electrostatic interference seen in the measurement.
Magnetic fields
A magnetic field passing through a loop in a test circuit generates a magnetic EMF (voltage) that is
proportional to the strength of the field, the loop area, and the rate at which these factors are
changing. Magnetic fields can be minimized by following these guidelines:
Locate the test circuit as far away as possible from such magnetic field sources as motors,
transformers, and magnets.
Avoid moving any part of the test circuit within the magnetic field.
Minimize the loop area by keeping leads as short as possible and twisting them together.
Electromagnetic interference (EMI)
The electromagnetic interference characteristics of the Model 6517B Electrometer/High Resistance
Meter comply with the electromagnetic compatibility (EMC) requirements of the European Union as
denoted by the CE mark. However, it is still possible for sensitive measurements to be affected by
external sources. In these instances, special precautions may be required in the measurement setup.
Sources of EMI include:
Radio and TV broadcast transmitters.
Communications transmitters, including cellular phones and handheld radios.
Devices incorporating microprocessors and high-speed digital circuits.
Impulses sources as in the case of arcing in high-voltage environments.
4-38
Model 6517B Electrometer Reference Manual
6517B-901-01 Rev. C / August 2015
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