https://en.m.wikipedia.org/wiki/Post_office_box_(electricity)gneill said:What is "post office box" (or "vox")?
Who says it can't be used to measure high or low resistance ?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.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?
A meter bridge, also known as a Wheatstone Bridge, is an instrument used to measure an unknown electrical resistance by balancing two legs of a bridge circuit. It consists of a one-meter-long wire of uniform cross-section, a galvanometer, a known resistor, and a jockey to make contact at different points on the wire. By adjusting the position of the jockey, the bridge can be balanced, and the unknown resistance can be calculated using the known values and the principles of the Wheatstone Bridge.
A meter bridge is preferred for measuring low resistance because it provides high accuracy and precision. The long wire used in the meter bridge allows for fine adjustments, which leads to more accurate determination of the balance point. Additionally, the meter bridge minimizes errors due to contact resistance and lead resistance, which are significant factors when measuring low resistances.
Using a meter bridge has several advantages: it offers high precision and accuracy, it is relatively simple and inexpensive to set up, and it allows for direct measurement of resistance without needing complex calculations or equipment. Furthermore, it minimizes errors associated with contact resistance and lead resistance, making it particularly suitable for low resistance measurements.
While the meter bridge is highly effective for measuring low resistance, it does have some limitations. It requires careful calibration and precise adjustments to ensure accuracy. The accuracy can also be affected by temperature variations and the uniformity of the wire. Additionally, it may not be suitable for extremely low resistances (in the milliohm range) without additional modifications or more sensitive equipment.
To improve the accuracy of low resistance measurements using a meter bridge, ensure that the wire used is of uniform cross-section and has good conductivity. Calibrate the instrument carefully before use, and make sure the connections are clean and secure to minimize contact resistance. Using a sensitive galvanometer can also help detect smaller imbalances, leading to more precise measurements. Additionally, conducting measurements in a controlled environment to minimize temperature fluctuations can further enhance accuracy.