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The answer is from google but I don't get it. Can someone explain in more simple way?

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You are mixing things up, the text talks about a galvanometer which is an electomechanical device which draws its power from the circuit in which it is used.

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When the currents become too small (less than a few micro Ampere) a galvanometer cannot be used anymore as you will not see the needle move anymore.

A Wheatstone Bridge:

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Is a circuit which can be used to determine the ratio of values of components. As such it is suitable also for high value resistors. What is required though is a sensitive volt/current meter in the middle.

A galvanometer is not that sensitive so a Wheatstone Bridge using a galvanometer is limited to resistor values up to around maybe 1 Mega ohm.

Replace the galvanometer with a more sensitive (electronic) meter (which could still have a galvanometer for readout) and you can use the Wheatstone Bridge also with high resistor values.

So the limitation is not due to the Wheatstone bridge but due to the galvanometer.

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