119 - A mystery eBay find: What is this box for?

I found this interesting wooden box on eBay for next to nothing. Especially intriguing was the meter with a scale reading Roentgens/hour.
It turned out to be somebody's vintage electronics project. An attempt to make a Wheatstone bridge to measure resistors, no xrays! The maker simply repurposed an old meter probably from an old xray machine. In the video I go over the design and implementation because there are a lot of strange things with the way it was made. I also explain how it works (sorry some maths...) and determine the value of some test resistors with it.

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0:00 The unit
1:51 Inside
4:02 The meter
7:44 Oddities
10:17 Schematic
11:46 How it works
16:51 Fixing the scales
17:27 Measuring resistors
21:51 Use as decade
22:14 Accuracy
22:53 Now what?

[graemebrumfitt]

Nice find, I love these old bits of kit! TFS, GB :) (Thanks For Sharing, Graeme Brumfitt)

[CXensation]

It's basically a Wheatstone Bridge Ohm meter.
R1 and R2 serves as bridge sensitivity.
You adjust R3 to match Rx.
With zero deflection on the Meter, Rx = R3.
Why it has a Röntgen range on the scale must be related to the dose sensor used.
Probably the meter was used for monitoring the Röntgen dose with a remotely operated range selector.
The bridge adjustment would then be a calibrator for the sensor.

[retireeelectronics]

Nice video. The box could be repurposed into an oversized clock etc, it looks very nice condition. That analog meter is probably a little treasure especially if you can set it up so the back can be rotated. Very nice.

[weinihao]

This probably was built by someone in a university lab for some project and then forgotten and moved into storage. Because machine shop (which likely supplied the case) turnaround times can be quite high, maybe the operator didn't bother to ask for proper alignment of the scales.

[cambridgemart]

Eastwood is a town in Hampshire, England; it's near to a number of big British electronic / aerospace / military companies, so probably many electronic surplus companies as well; I would guess P & H were one of the surplus companies and, like many others sadly, long gone.

[td]

I wonder if it was a commercial educational kit? Maybe a couple of resistors were left out of the kit and were never sourced and installed? I also wonder if the red button was wired correctly, as the value of its two resistors seems arbitrary. Can it also be used as a nulling meter? Anyway, all good fun.

[BjornV]

i guess cleaning all the switch contacts, will also improve the accuracy. Nice find of Ebay.

[TheDefpom]

Re-pack it with higher wattage 0.1% resistors… then it will truly be a great bit of equipment worth keeping.

[ivolol]

I would think that replacing the R1 and R2 resistors with some modern 0.1% resistors, or 1%'s that are hand-binned, might improve the accuracy of it quite a lot. General multimeters are usually not great at low ohm measurements, so especially if you could improve the accuracy towards that range, it might be a more valuable bit of kit. I don't quite get the 'red button resistors'... if they are switched in, wouldn't they upset the ratio intended to calculate with R1 & R2?

If nothing else, an adjustable resistor comes in handy once in a blue moon.

[FindLiberty]

A Bridge over troubled radioactive waters?

[philipdouglas]

Rather than destroy the authenticity of the thing by replacing the resistors, how about accurately measuring their values, and then setting up a spreadsheet to give an accurate value based on their measured values?

[KetkAiball]

Hello again. Today night the following videos are ready to watch:
It was a time these kind of bridges instruments were very useful. People use to make them homemade. They were very skillfull people. And why they were as useful instruments? because they had the highest precision for measuring resistor values. Much higher than the best analog ohmmeter.
Greetings.
Ketk Aiball.

[johncoops]

Almost certainly a kit, or based upon one. I grew up in the 1970s and it was very common for small companies to advertise "parts kits" like this in electronics magazines, so the £25 may have been a set of switches and precision resistors. YouTube keeps deleting my comment and I cannot work out what word(s) is triggering it. 😀😀

[reedreamer]

Is this an electrical instrument or a game consoled? ; )