Post Office Box Plug Type
Product Code : LSK-EAE-11113
Post Office Box Plug Type (SKU LSK-EAE-11113), or P.O. box, is a compact Wheatstone bridge in a wooden box: two ratio arms and a rheostat arm of plug-selected coils let students balance a bridge and find an unknown resistance with a DC battery and galvanometer. It is made in 3-ratio and 4-ratio versions for Class 12 and B.Sc. physics labs. Science Lab Supplies manufactures it in its Saha, Ambala works (Plot 448, HSIIDC Industrial Area) and supplies schools, colleges, tender contractors and distributors directly.
Specifications
The table lists every specification of both versions. In short, the 3-ratio box has sixteen non-inductive coils from 1 to 5000 Ω with ratio arms of 10, 100 and 1000 Ω and an infinity gap; the 4-ratio box adds 1 Ω ratio coils and measures 0.1 to 1111000 Ω. Manganin or constantan coils are offered.
| Specification | Value |
|---|---|
| SKU | LSK-EAE-11113 |
| Versions | 3-ratio and 4-ratio |
| Rheostat arm coils | Sixteen non-inductively wound coils: 1, 2, 2, 5, 10, 20, 20, 50, 100, 200, 200, 500, 1000, 2000, 2000, 5000 Ω |
| Ratio arms, 3-ratio | Two arms, each with 10, 100, 1000 Ω coils |
| Ratio arms, 4-ratio | Two arms, each with 1, 10, 100, 1000 Ω coils |
| Infinity gap | Incorporated (3-ratio) |
| Measuring range, 4-ratio | 0.1 to 1111000 Ω, using a DC battery |
| Keys, 4-ratio | Two spring keys, for battery and galvanometer |
| Brass block | Rectangular, 12 mm and 14 mm thick |
| Accuracy, manganin coil | ±0.05% |
| Accuracy, constantan coil | ±0.1% |
| Enclosure | Wooden box |
| Manufacturer | Science Lab Supplies, Saha, Ambala, Haryana |
| Country of origin | India |
Applications and Experiments for the Post Office Box
The P.O. box puts all four arms of a Wheatstone bridge into one unit. Students set a ratio, adjust the plugs in the third arm until the galvanometer shows no deflection, and calculate the unknown resistance, a classic exercise in physics lab equipment list class 11 and 12 courses.
- Class 12 physics: resistance of a wire and its specific resistance, with a Galvanometer as the null detector.
- Verifying the laws of resistances in series and parallel.
- B.Sc. practicals: low and high resistances across the 4-ratio range, closing the circuit briefly with a Tapping Key Physics Lab.
- Powering the bridge from cells or a Laboratory Battery Eliminator.
Press the battery key before the galvanometer key and release them in reverse order, and keep all plugs clean and tight.
Standard Scope of Supply
The Post Office Box Plug Type is supplied as a single unit in its wooden box, in the 3-ratio or 4-ratio version and with manganin or constantan coils as ordered. The battery, galvanometer and connecting leads are not included and can be quoted separately.
Packing, MOQ and Lead Time
You can request one P.O. box or a set for every bridge experiment station, naming the version and coil type. Science Lab Supplies quotes packing, delivery time and price for that quantity, and galvanometers, keys or resistance boxes can be added.
With a registered Importer-Exporter Code, Science Lab Supplies can ship to buyers in Ethiopia, Tanzania, Uganda, the UAE, Saudi Arabia and Oman. Submit your requirement on the contact page or in the enquiry cart, and use the lab tender page for tender supply.
Frequently Asked Questions
What is the difference between the 3-ratio and 4-ratio Post Office Box?
Both have the same sixteen-coil third arm. The 3-ratio box has ratio coils of 10, 100 and 1000 Ω plus an infinity gap; the 4-ratio box adds a 1 Ω coil to each ratio arm and measures 0.1 to 1111000 Ω.
How accurate are the coils?
Boxes with manganin coils are accurate to ±0.05%, and boxes with constantan coils to ±0.1%. Both use rectangular brass blocks 12 mm and 14 mm thick.
Why is it called a Post Office Box?
The design takes its name from the bridges once used by telegraph engineers to locate faults in lines. In the laboratory it works as a Wheatstone bridge built into one box.
Is Science Lab Supplies the manufacturer or a trader?
Science Lab Supplies manufactures the P.O. box in-house at Saha, Ambala, India. Coil winding, brass work and assembly take place there, and quotations are sent by the manufacturer itself.
Related Bridge and Resistance Apparatus
These Electrical and Electronics, Applied Mechanics and Lab Meters products serve the same bridge and resistance practicals, from the meter bridge and resistance boxes to a bridge board and sensitive galvanometer, so a full bridge bench for the laboratory can be quoted together.
- Meter Bridge or Wheatstone Bridge With Pencil Jockey — four-gap bridge with 24 SWG constantan wire on a metre scale.
- Resistance Box Plug Type (Constantan / Manganin Coil) — plug box with non-inductive coils and brass block.
- Decade Resistance Box, Dial Type — 1% metal film decades in two, three or four dials.
- Resistance Substitution Box — 24 steps from 10 Ω to 1 MΩ.
- Portable Wheatstone Bridge Apparatus Circuit Board — four-arm bridge with centre-zero galvanometer and 9 V battery.
- Laboratory Spot Reflecting Galvanometer — light-spot indicator for detecting small currents.
- Resistance Coil Unit 1 Ohm — 1 Ω ±1% manganin standard on a base plate.
Browse the full Electrical and Electronics range, the Lab Meters range for galvanometers, or all physics lab equipment.
Last updated: 9 October 2026
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