TENTECH MD1035x Manual De Uso página 28

In the circuit of fig. 3 it may be noted that R2 is in parallel with a low-value
resistance (the one from the micro-ammeter) therefore its influence is
reduced during reading.
Through resistance R1 circulates a current which is not passing through
the meter and consequently does not affect the reading. In fact, current
through R1 originates a certain error, since it creates an additional voltage
drop, but it can be considered completely insignificant (especially in digital
equipment).
For all practical purposes, the Guard terminal allows to measure the value
of Rx with an insignificant error.
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