Wheatstone bridge to measure high resistances?

AI Thread Summary
A Wheatstone bridge can measure high resistances, but accuracy diminishes due to current limitations through the resistor being tested. The challenge arises when trying to detect a zero balance point, as the current may be too low for precise readings. Solutions include using amplifiers or replacing the galvanometer with a low-offset op-amp to improve measurement capability. For resistances around 10^8 ohms or higher, specialized equipment like a low input bias current transimpedance amplifier or instruments from Keithley Instruments is recommended. Overall, while a Wheatstone bridge can be adapted for high resistance measurements, it requires careful consideration of its limitations.
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Can a wheatstone bridge be used to measure very high resitances with good accuracy? If not what problems interfere?
 
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One problem would be that the current for the meter that detects the balance point has to come through the resistor being measured.

So, you might be trying to detect when the meter is actually reading zero instead of some other very low value of current.

There are ways around this (by adding amplifiers for example) but the basic Wheatstone Bridge would have such difficulties.
 
It has been a long time since I had to use a Wheatstone bridge. I think the highest resistance in it was about 1 megohm. The galvanometer does not carry any current when the bridgre is balanced, but the high-meg resistor R would have to carry ~1 volt/R amps, and the galvanometer might have a full scale deflection of ? 1 microamp ?. So the galvanometer should be replaced with a low-offset op amp. For 10^8 ohms or higher I would either build a low input bias current/low voltage offset inverting transimpedance amplifier, or buy an instrument from Keithley Instruments.

Bob S
 

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