Post Office Box Physics || Current Electricity Class 12, JEE & NEET
Post Office Box is a Wheatstone bridge-based device used to measure unknown resistance, specific resistance, internal resistance of a cell, and galvanometer resistance by balancing known ratio arms and a variable resistance arm.
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A Post Office Box can also be used to measure an unknown resistance. It is a Wheatstone Bridge with three arms P, Q and R; while the fourth arm(s) is the unknown resistance. P and Q are known as the ratio arms while R is known as the rheostat arm. Post Office Box, Physics named it so because it has a shape of box and was designed to find resistance of electric cables and telegraph wires. It was used in post offices to determine resistance of transmission lines.
- At balance, the unknown resistance S = (P/Q) R …… (1)
- It is based on principle of Wheatstone bridge.
- Unknown resistance is $\mathrm{S}=\frac{\mathrm{Q}}{\mathrm{P}} \mathrm{R}$ and specific resistance is $\rho=\frac{\pi r^{2} S}{L},$ where $r$ is radius and $L$ is length of wire.
- In Post Office box we first press cell key and then press galvanometer key to eliminate induced effects.
- It is used to find unknown resistance, specific resistance of a wire, internal resistance of cell, resistance of galvanometer etc.
- A typical post office box is in a wooden box with a hinged lid and a metal or Bakelite panel showing circuit connections. Coils of wire are wound non-inductively, mounted in the body of the box, and have a negligible temperature coefficient. Pairs of ratio arms are each 10, 100, 1000 ohms. A resistance arm contains a number of coils from 1 to 5000 ohms with a plug for infinite resistance.
Physics Revision Series by Saransh Sir (AIR 41 in IIT-JEE) Also Read:
- Types and Effects of Electric Current
- Ohm’s Law and Resistance
- Combination of Resistances
- EMF and Internal Resistances of a Cell
- Cells Connected in Series, parallel and Mixed
- Kirchhoff’s Circuit Law
- Electric Currents in Conductors
- Wheatstone Bridge
- Post office Box
- Wheatstone Meter Bridge
- Moving Coil galvanometer
- Ammeter and Voltmeter
- Potentiometer Working Principle
Frequently Asked Questions
Find answers to common questions.
What is a Post Office Box in physics?
A Post Office Box is a precise resistance-measuring instrument based on the Wheatstone bridge principle. It has three internal arms — P and Q (ratio arms) and R (rheostat arm) — and the unknown resistance S forms the fourth arm. At balance (null galvanometer reading), the unknown resistance is calculated as S = (Q/P) × R. It was originally used by post offices to test telegraph cable resistance.
What is the formula for unknown resistance in a Post Office Box?
The formula is S = (Q/P) × R, where P and Q are the ratio arm resistances (selected from 10, 100, or 1000 Ω), and R is the rheostat arm resistance at the null point. This is derived directly from the Wheatstone bridge balance condition P/Q = R/S, rearranged to solve for S.
Why is the cell key pressed before the galvanometer key in a Post Office Box?
The cell key is pressed first to allow steady current to flow through the circuit. If the galvanometer key is pressed first, the changing magnetic flux during current build-up induces an EMF that deflects the galvanometer falsely. Pressing the cell key first eliminates this induced effect and gives a reliable null-point reading.
What is the difference between a Post Office Box and a Meter Bridge?
Both are Wheatstone bridge instruments, but the Post Office Box uses plug-selected coils for its ratio and rheostat arms, giving higher precision over a wider resistance range. A Meter Bridge uses the position of a jockey on a uniform resistance wire to set the ratio. Post Office Boxes are bulkier but more accurate; Meter Bridges are simpler and commonly used in school labs.
How is specific resistance calculated using a Post Office Box?
After measuring the unknown resistance S of a wire using the Post Office Box, specific resistance is calculated using ρ = πr²S/L, where r is the radius of the wire (measured using a screw gauge) and L is the length of the wire. This formula assumes the wire is uniform and its cross-section is circular.