Measuring Low Resistance: Why Meter Bridge is the Preferred Choice

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Discussion Overview

The discussion revolves around the comparison between meter bridges and post office boxes (Wheatstone bridges) for measuring low resistance. Participants explore the reasons for preferring meter bridges, the limitations of post office boxes, and the underlying principles of these measurement techniques.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question why meter bridges are preferred over post office boxes for measuring low resistance.
  • There is a request for clarification on what a "post office box" is, with references to it being a Wheatstone bridge circuit.
  • One participant suggests that the post office box is primarily used to find short circuits rather than measuring resistance.
  • Another participant proposes that the measurement range of the post office box is limited by the available ratio arms and standard resistances, affecting its effectiveness for low resistance measurements.
  • A method for measuring low resistances is mentioned, where a current is passed through the meter for full-scale deflection before measuring the drop when shunted by the unknown resistance.

Areas of Agreement / Disagreement

Participants express differing views on the suitability of post office boxes for measuring resistance, with no consensus reached on their effectiveness compared to meter bridges.

Contextual Notes

Limitations include the dependence on the specific configurations of the measurement devices and the potential for unresolved technical details regarding their operational principles.

phymath7
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TL;DR
Why post office box can't be used to measure high or low resistance?
Why meter bridge is chosen over post office box to measure low resistance?
 
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What is "post office box" (or "vox")?
 
Ah. According to the Wikipedia article the Post Office Box (origin UK) is in fact a Wheatstone bridge circuit.
 
phymath7 said:
TL;DR Summary: Why post office box can't be used to measure high or low resistance?

Why meter bridge is chosen over post office box to measure low resistance?
Who says it can't be used to measure high or low resistance ?
The post office box was two arms of a Wheatstone bridge.
It was used to find short circuits, not open breaks in the line.
 
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phymath7 said:
TL;DR Summary: Why post office box can't be used to measure high or low resistance?

Why meter bridge is chosen over post office box to measure low resistance?
I think the measurement range will be limited by the available ratio arms and standard resistances. In addition, a bridge does not make best use of the meter sensitivity because it is shunted by the bridge components. There is a sensitive method for low resistances where we first pass a current through the meter to give full scale deflection and then measure the drop in reading when the meter is shunted by the unknown low resistance.
(Sorry, I recently disposed of my copy of "Telephony", by Herbert and Proctor, which would have given the definitive answer).
 

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