Post Office Box Plug Type
Product Code : EL-EAE-10285
Quick answer: The Post Office Box Plug Type is a compact Wheatstone bridge in a wood box, used with a DC battery and galvanometer to measure resistances from 0.1 to 1,111,000 ohms. It is made in 3-ratio and 4-ratio versions with manganin (±0.05%) or constantan (±0.1%) coils. Lab Exports manufactures it in Delhi for schools, colleges and universities.
Specifications
| Item | Specification |
|---|---|
| Measurement range | 0.1 to 1,111,000 ohms |
| 4-ratio version | 16 coils; two ratio arms with 1, 10, 100 and 1000 ohm coils each; infinity gap |
| 3-ratio version | 16 non-inductive coils; two ratio arms with 10, 100 and 1000 ohm coils each |
| Accuracy, manganin coils | ±0.05%, 12 mm / 14 mm thick brass block |
| Accuracy, constantan coils | ±0.1%, 12 mm / 14 mm thick brass block |
| Housing | Wood box |
How the Post Office Box experiment is done
A Post Office Box contains the three known arms of a Wheatstone bridge: two ratio arms P and Q and a third arm R made up of 16 coils. The unknown resistance X forms the fourth arm, with a battery across one diagonal and a galvanometer across the other. When the galvanometer shows no deflection the bridge is balanced and P/Q = R/X, so X = (Q/P) R.
Choosing unequal ratio arms extends the range and sensitivity. With P = 10 and Q = 10 the balance gives X directly; with P = 100 and Q = 10, or P = 1000 and Q = 10, R is set ten or a hundred times larger than X, so X can be found to one or two extra decimal places.
- Connect the unknown resistance, a battery with key and a galvanometer with key to the terminals of the box.
- Take out 10 ohm plugs in both ratio arms and find the two values of R between which the deflection reverses.
- Change the ratio to 100:10 and then 1000:10, narrowing the range each time.
- For each trial press the battery key before the galvanometer key to avoid induced kicks.
- Calculate X = (Q/P) R from the final balance.
Key features
- Measures resistance from 0.1 to 1,111,000 ohms
- 4-ratio version with 1, 10, 100 and 1000 ohm ratio arms and an infinity gap
- 3-ratio version with 10, 100 and 1000 ohm ratio arms and non-inductive coils
- Manganin coils to ±0.05% or constantan coils to ±0.1%
- Thick brass block and wood box for a compact bridge on the bench
Documentation and supply
Most Lab Exports products carry a one-year manufacturer warranty.
Lab Exports manufactures the Post Office Box at its works in Laxmi Nagar, Delhi, and supplies it to schools, colleges and universities in India and 60+ countries, handling the export documents for each shipment. More about the company is on the About Lab Exports page.
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The full range is in the Electrical and Electronics section of the Physics Lab range.
Frequently asked questions
What is the range of the Post Office Box?
It measures resistances from 0.1 to 1,111,000 ohms using a DC battery and galvanometer.
What is the difference between the 3-ratio and 4-ratio versions?
The 4-ratio version has ratio arm coils of 1, 10, 100 and 1000 ohms plus an infinity gap; the 3-ratio version has ratio arm coils of 10, 100 and 1000 ohms.
What coil options are available?
Manganin coils with ±0.05% accuracy or constantan coils with ±0.1% accuracy.
What is the price of the Post Office Box?
Prices are not published on the website. Add the Post Office Box to the Enquiry Cart with the Add To Cart button on this page, or email [email protected] with the house code EL-EAE-10285 and the quantity, to receive a quotation.
Does Lab Exports manufacture the Post Office Box?
Yes. Lab Exports manufactures the Post Office Box at its works at 11/315, Lalita Park, Laxmi Nagar, Delhi 110092, under house code EL-EAE-10285.
Request a quote
Email [email protected] with the house code EL-EAE-10285 and the quantity, or add the Post Office Box to the Enquiry Cart with the Add To Cart button on this page. Tender buyers can also see the Tenders and OEM page.
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