18 Glossary
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18.20
Four-wire measurement
(Kelvin measurement)
If an current flows through an electric conductor a
voltage will drop due to the specific resistance of the
conductor. This interrelationship is known as Ohm's law
(R=V/I). The voltage at the and of the wire is lower as at
the beginning of the wire (the point of measurement).
To get the exact voltage at the point of measurement
anyway the four-wire method is used: here the current flows through two of the four wires where the input
resistance of the measurement device is low (here voltage will drop on the wire), the voltage is measured over
the other two wires that are connected to a high resistance input (here nearly no current flows and thus no
significant voltage drops).
18.21
Efficiency
General: Relationship of the usable to the assigned energy. For illustration: Conventional bulbs transform about
3 - 4% of the assigned energy into light, photovoltaic generators respectively solar cells at present reach an
efficiency of 11 - 17%, thermal solar plants between 25 and 40% regarding the solar radiation.
Operation manual for PVPM devices
PV-Engineering GmbH
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