Post Office Box experiment

So, for a bit of completely pointless fun really, does this antique looking item that was acquired entirely for novelty/aesthetics value work, what is it for, and how accurate is it?

Until very recently I had no answer to any of those questions. It caught my eye at a car boot sale and I liked it. It just sat there looking cool, then we moved house, and it went missing. Fast forwards a year or two and some of the guru's on here started to talk about using a Wheatstone bridge to accurately measure a resistance and help with underground cable faults. The penny then dropped what it was actually for. A search through boxes finally found it and I felt compelled to try it out.



In a nutshell the principle is that the box has 3 variable resistors (top left, top right, and bottom three rows). The fourth resistance is the cable loop you want to measure.
When [top right] divided by [top left] = [cable loop resistance] / [bottom three rows] the voltage accross the middle will be zero.
So you effectively set the scale with the top row, 10 / 1000 in this case, and adjust the bottom three rows until you get as close to zero volts as possible.

My test was to try measure the resistance of this short bit of alarm cable:


The setup goes like this:


You set a value be removing pegs ( some are unfortunately missing so some holes are plugged with bolts).
A DC power supply of up to about 40v (I think) is connected to + and - . The left button connects the power, and the right button connects the measuring terminals to the middle of the bridge.

Here's a poor photo of the actual test with a 12v power supply and a voltmeter connected showing the bridge (virtually) balanced.



When a 10 and a 2 were removed from the bottom three rows, the bridge was very close to zero volts. So a value of 12 ohms balances the bridge.
The top row were set on 10 ohms and 1000 ohms.
So 10/1000=X/12
Or X=10/1000 * 12 giving a final result of 0.12 ohms.

So how did it compare to a calibrated MFT?


I was quite surprised, only 0.01 ohms difference. Not bad at all!
It can go back on it's shelf now and look cool again with my respect for it having increased a bit.

But did you manage to get a tune out of it?

I was taught about these in collage in the 70s but never actually used one. I think there were a couple of slightly different designs. A varley bridge was used for power cables and a murry bridge for telecoms.

timhoward said:

I was quite surprised, only 0.01 ohms difference. Not bad at all!

Click to expand...

Which is only one count on the digital display and all digital displays are allowed +/- 1 count in addition to the instrument accuracy, sometimes more. So the difference might be yet smaller.

The important difference between the two measurement techniques is that a direct-reading method relies on the accuracy of the meter as well as its external scaling circuit, whereas a bridge method relies only on the bridge components themselves being accurate. Provided the meter used to detect the null is sufficiently sensitive and can be zeroed, the accuracy of its calibration is unimportant. If all the bridge resistors are wire wound using the same alloy, reliable, stable calibration can be achieved that is near independent of temperature.

Many electronic measurement systems that detect tiny changes rely on bridge measurement. Strain gauges are a good example of a resistive bridge where the deflection varies the resistance of the elements only fractionally, possibly much less than the variations due to temperature and time; but the bridge configuration allows the differential measurement to remain accurate despite the interfering factors.

I love things like this, so very satisfying to get a result like you got too! Ive never seen one in my life! I too though undoubtedly would have ended up buying it!

You could try a more modern one BT Metrohm Bridge Ohmmeter 18C/2 Cable Fault Tester | eBay - https://www.ebay.co.uk/itm/294134130668