Test Equipment Anonymous

I've already got dotted-LED, VFD, Panaplex, Nixie, Dekatron displays, but haven't got any projection displays - until now.

I was given them under the pretence they were digits, and so was planning on making an old-skool DVM But that's not to be: they don’t display digits! Instead they show units such as “mega” “centi” “milli” “W” “A” “V” “Wh” “VAr” “bar” “Kg/s”[sic] and others. Clearly they are part of a process control display. It isn’t a problem for me; if anything it makes them more endearing.

The light bulbs are mostly 24V 2.8W and take 100mA, but one takes 140mA. About 10% have failed. Given the light is projected onto ground glass, they have wide angle readability. The low means they aren’t bright; I expect a modern LED would address that issue.

Very pretty. Shame about the "Kelvin, g, s" legend; it must be American. I have a (very mild) hankering for a torsion wire galvanometer as used at school with its illuminated line moving a glass display.

EC8010 wrote: ↑Sun Feb 01, 2026 7:09 pm Very pretty. Shame about the "Kelvin, g, s" legend; it must be American.

Apparently it came from Hinckley Point A when it was decomissioned.

I have a (very mild) hankering for a torsion wire galvanometer as used at school with its illuminated line moving a glass display.

Yes, me too. Used them as a ballistic galvanometer to measure relative charge.

The chances of getting one where it was stored and shipped in the locked position seem slim; damage seems inevitable.

Same is true with saturated Weston Standard cells. You sometimes see those on fleabay, in which case I tell the seller of the shipping hazard (especially with aluminium in aircraft).

EC8010 wrote: ↑Sun Feb 01, 2026 7:09 pm Very pretty. Shame about the "Kelvin, g, s" legend; it must be American. I have a (very mild) hankering for a torsion wire galvanometer as used at school with its illuminated line moving a glass display.

We had a Pye mirror galvanometer/µ ammeter. The physics teacher showed us the magnet in coil demonstration and by pushing the magnet in too quickly, drove the spot off the scale so much it took a time to recover. I've seen a few of them around since but have never been tempted because I have no use, they are bulky, and they are usually tatty. In the text books there were older and more awkward designs. All ebonite and brass with little adjusting feet to level them, and a glass bell to keep the dust off.

AFAIK they didn't have a torsion wire, it was a thread of quartz. The method of making it was interesting and involved a crossbow. There was a bolt with a bar of quartz. The bar was heated in the right place with a gas flame, and at the right moment the bolt was shot and went off trailing the precious thread.

Zenith wrote: ↑Sun Feb 01, 2026 9:46 pm I've seen a few of them around since but have never been tempted because I have no use, they are bulky, and they are usually tatty.

AFAIK they didn't have a torsion wire, it was a thread of quartz. The method of making it was interesting and involved a crossbow. There was a bolt with a bar of quartz. The bar was heated in the right place with a gas flame, and at the right moment the bolt was shot and went off trailing the precious thread.

Yes, proper tatty. And of no real use. Likewise, I passed by a current balance a few years ago.

You're having me on! Surely?

I had forgotten of the danger of mercury rotting aluminium. I have a (Tinsley) Weston cell that is still the expected voltage.

The quartz thing seems unlikely, but what do I know. Why is quartz thread ideal, how did they determine that, and who dreamed up that magic sauce?!

One of mine, still in spec after three quarters of a century

EC8010 wrote: ↑Sun Feb 01, 2026 11:12 pm

Zenith wrote: ↑Sun Feb 01, 2026 9:46 pm I've seen a few of them around since but have never been tempted because I have no use, they are bulky, and they are usually tatty.

AFAIK they didn't have a torsion wire, it was a thread of quartz. The method of making it was interesting and involved a crossbow. There was a bolt with a bar of quartz. The bar was heated in the right place with a gas flame, and at the right moment the bolt was shot and went off trailing the precious thread.

Yes, proper tatty. And of no real use. Likewise, I passed by a current balance a few years ago.

You're having me on! Surely?

I had forgotten of the danger of mercury rotting aluminium. I have a (Tinsley) Weston cell that is still the expected voltage.

I kid you not.

W. Littler "A New School Chemistry" page 381.

https://archive.org/details/newschoolch ... 2/mode/2up

Quartz threads could be made very thin and unwound perfectly after torsion.

The crossbow only shot the bolt about 30 feet, so it wasn't a medieval monster with an elaborate cranking system and a half ton draw weight.

Talking of mercury attacking aluminium, Auguste Piccard did ascents into the stratosphere in a balloon with a pressurised aluminium gondola. On one of these flights a barometer shattered and the mercury splashed onto the floor. They realised it would eat through the floor very quickly.
Time for quick thinking. They had a length of rubber pipe connected to the outside via a valve. They used the pipe to suck up the mercury and fling it outside.

I read it in a comic when I was a kid, but I checked and it did happen.

https://www.sciencehistory.org/stories/ ... 5000-feet/

Zenith wrote: ↑Mon Feb 02, 2026 6:14 am W. Littler "A New School Chemistry" page 381.

https://archive.org/details/newschoolch ... 2/mode/2up

Quartz threads could be made very thin and unwound perfectly after torsion.

The crossbow only shot the bolt about 30 feet, so it wasn't a medieval monster with an elaborate cranking system and a half ton draw weight.

Fascinating!

That book also describes the method I used to stink out the kitchen at home. My father brought home the conc HCl in a plastic bottle on the bus
https://archive.org/details/newschoolch ... 5/mode/1up

It doesn't, however, mention two experiments we did at school: adding conc H₂SO₄ to sugar, and adding metallic Na to water. EDIT: see p296, but only in passing.

Overall it feels like my memories of chemistry, and why I only took it to A-level. It is a lot of isolated phenomena of interest due to the industrial importance and/or analytic importance. I think chemistry has become less analytic and more synthetic since then, but I don't really know.

Talking of mercury attacking aluminium, Auguste Piccard did ascents into the stratosphere in a balloon with a pressurised aluminium gondola. On one of these flights a barometer shattered and the mercury splashed onto the floor. They realised it would eat through the floor very quickly.
Time for quick thinking. They had a length of rubber pipe connected to the outside via a valve. They used the pipe to suck up the mercury and fling it outside.

I read it in a comic when I was a kid, but I checked and it did happen.

https://www.sciencehistory.org/stories/ ... 5000-feet/

After a pleasant flight (superb cloudscapes) back from CERN I was chatting with the pilot. One of his anecdotes was that he found one of his cargo boxes was leaking water. He found it contained elvers, so the question was whether it was salt or fresh water. That's relevant since salt water will subtly alter aluminium. It was fresh water, so the flight too place without a trip to the engineering department.

Thank you for that. I can just imagine the response had I said that I needed to make some fine quartz thread at work. The blow lamp would have been a minor problem compared to the crossbow! Another useless fact to be stored away.

Many years ago, there was a pick-up arm made by KMAL (Keith Monks Audio Limited). At the time, low effective mass and ultra low friction were the way to go, so unipivots were popular (especially as you can make a unipivot arm quite easily if you have a lathe). But you needed really fine wire to avoid adding significant friction/stiction and that wasn't easily available. So the KMAL arm had four little wells in an insulating material, filled with mercury. Into these wells dipped gold-plated pins, thereby giving a low friction contact. Even I know that mercury and gold isn't a good combination. The arm would undoubtedly have been made of aluminium. And then we get to exposed mercury in the home...

EC8010 wrote: ↑Mon Feb 02, 2026 9:56 am And then we get to exposed mercury in the home...

... and during school chemistry lessons, and in dental fillings, and a surprsing range of seafood
https://www.nhs.uk/live-well/eat-well/f ... nutrition/

We were "constantly" making "squeaky pops" at school to confirm presence of hydrogen. My daughter was barely aware of that technique.

tggzzz wrote: ↑Mon Feb 02, 2026 11:03 am

EC8010 wrote: ↑Mon Feb 02, 2026 9:56 am And then we get to exposed mercury in the home...

... and during school chemistry lessons, and in dental fillings, and a surprsing range of seafood
https://www.nhs.uk/live-well/eat-well/f ... nutrition/

Mercury was a mainstay of school labs. I remember an experiment which demonstrated that germinating seeds did not need oxygen. A few soaked peas were put in a test tube full of mercury inverted in a bowl of mercury. After a day or two gas had accumulated which was shown to be CO2. I doubt the biology master wore a hasmat suit when he set it up.

Then there was Barlow's Wheel which I saw in textbooks but I doubt any school labs had one.

https://nationalmaglab.org/magnet-acade ... heel-1822/

There was another one with a magnet in a bowl of mercury and a loosely pivoted metal rod. When the current flowed in one direction the rod circled the magnet clockwise, and if the magnet was reversed or the current reversed, it went anticlockwise.

The switches to desensitise meter bridges had two small wells of mercury into which a resistor or a wire could be placed. It seemed unnecessary at the time.

At home there were mercury thermometers and barometers. Fluorescent lights contained an amount of mercury. I heard that in the early days they used beryllia as a phosphor. Mercury filled tilt switches etc.

tggzzz wrote: ↑Mon Feb 02, 2026 11:03 am We were "constantly" making "squeaky pops" at school to confirm presence of hydrogen. My daughter was barely aware of that technique.

That was in the days when you could enter a school lab without goggles, ear defenders and full body armour. I once saw it done with acetylene. There was a loud bang and the inside of the tube was coated with soot. It's a wonder the test tube didn't shatter.

Of the things I saw in text books and wanted.

A Wimshurst machine. I could buy or make one now. What's the point?

An induction coil. The same.

A mirror galvanometer. I would have wrecked it very quickly.

A Fortin barometer. On second thoughts, not a great thing to have in the home.

A Weston Standard Cell. I have a few now. Back then I would have wrecked it by connecting a crappy multimeter and quite likely have broken it causing a chemical hazard.

A tangent galvanometer. That's something I might be tempted by if I saw one, which I never have.

EC8010 wrote: ↑Mon Feb 02, 2026 9:56 am Many years ago, there was a pick-up arm made by KMAL (Keith Monks Audio Limited). At the time, low effective mass and ultra low friction were the way to go, so unipivots were popular (especially as you can make a unipivot arm quite easily if you have a lathe). But you needed really fine wire to avoid adding significant friction/stiction and that wasn't easily available. So the KMAL arm had four little wells in an insulating material, filled with mercury. Into these wells dipped gold-plated pins, thereby giving a low friction contact. Even I know that mercury and gold isn't a good combination. The arm would undoubtedly have been made of aluminium. And then we get to exposed mercury in the home...

That idea is so daft it's a marvel it got onto the drawing board, leave alone to even the prototype stage.

I heard about an ionic loudspeaker (the ionofane??). It produced ozone, something not without hazards. One of the claimed advantages was that it killed mice in the room.

Zenith wrote: ↑Mon Feb 02, 2026 12:27 pm Of the things I saw in text books and wanted.

A Wimshurst machine. I could buy or make one now. What's the point?

Saw one over the weekend for £25, on fleabay. I would have bought it to test my 40kV meter, but it was incomplete.

An induction coil. The same.

I've got many of those. The first I used in anger was at school. I used PO3000 relay connected as a ring oscillator to disrupt nearby radios. The sparks were quite enough to give a jolt.

Zenith wrote: ↑Mon Feb 02, 2026 12:36 pm I heard about an ionic loudspeaker (the ionofane??). It produced ozone, something not without hazards. One of the claimed advantages was that it killed mice in the room.

Indeed, the Ionofane. A French invention marketed by Fane Acoustics. A pointed tungsten electrode fed by 27MHz produced a local corona and heat. Amplitude modulation of the RF modulated the heated volume, producing the perfect massless transducer. But the displacement was rather small, so only usable as a tweeter. Even so, there wasn't much output, and a horn was needed to achieve usable levels. The horn was die-cast aluminium. So we had a 10W amplitude modulated 27MHz RF source at the base of a conductive horn, pointed at the listener. What could possibly go wrong?

There are usually Wimshurst machines at the Audiojumble and especially at Retrotech. I always favoured van de Graaf generators; the school Wimshurst machine often didn't work. Probably not enough Perspex in its construction.

It occurs to me that a number of my teachers had probably been shot at during WWII, so sodium or calcium in water or spilled mercury were pretty small beer. I remember being impressed by physics master's demonstration of Crooke's tube where he was forcing the bungs in at each end to maintain a good vacuum, with 10kV applied end to end on ordinary banana leads... He had a slightly distorted face, and in hindsight, that might have been a bullet wound.

I believe some commercial KVDs recommended cleaning their terminals with mercury amalgam to get a good contact.

I used the principle of the tangent galvonometer to calibrate my Helmholtz coils using a compass and the Earth's magnetic field.

EC8010 wrote: ↑Mon Feb 02, 2026 2:18 pm I believe some commercial KVDs recommended cleaning their terminals with mercury amalgam to get a good contact.

There's always the "horned" standard resistors such as this one:



The downward pointing prongs were dipped in a mercury bath to make the connection.

I've got some of those! Never noticed the downward pointing prongs before...

EC8010 wrote: ↑Mon Feb 02, 2026 2:18 pm It occurs to me that a number of my teachers had probably been shot at during WWII, so sodium or calcium in water or spilled mercury were pretty small beer. I remember being impressed by physics master's demonstration of Crooke's tube where he was forcing the bungs in at each end to maintain a good vacuum, with 10kV applied end to end on ordinary banana leads... He had a slightly distorted face, and in hindsight, that might have been a bullet wound.

The demonstration with sodium or potassium in water in a large glass vessel had to be done with caution, using a bit of the metal no more than about 3mm diameter. I heard of a student teacher bunging a whole lump of sodium into a glass pneumatic trough full of water. It caused an explosion lacerating himself and several members of the class.

A physics teacher connected a Coolidge Tube to an induction coil and invited the class to get close so they could see the apple green glow. I retired to the far end of the class. He told me I couldn't see anything from there. I said I'd seen enough already. A more sensible and competent physics teacher had told me before that it was illegal to do that.

Zenith wrote: ↑Mon Feb 02, 2026 2:54 pm The demonstration with sodium or potassium in water in a large glass vessel had to be done with caution, using a bit of the metal no more than about 3mm diameter. I heard of a student teacher bunging a whole lump of sodium into a glass pneumatic trough full of water. It caused an explosion lacerating himself and several members of the class.

Yes indeed. The teacher cut very small slivers for us to "use". He also told a story about accidentally knocking a slightly larger piece into the sink, with a startling result.

First couple of A-level chemistry lessons consisted of the teacher bring out every bottle in the organic chemical cupboard, and asking us what they smelled of. Doubt that would happen nowadays.

Later an experiment used potassium cyanide. We were warned to clean it out in the fume cupboard, being sure there was no acid in the sink.

tggzzz wrote: ↑Sun Feb 01, 2026 6:20 pm I've already got dotted-LED, VFD, Panaplex, Nixie, Dekatron displays, but haven't got any projection displays - until now.

What's this, no minitron or magic eye? I'm shocked, and worried!

AVGresponding wrote: ↑Mon Feb 02, 2026 5:19 pm

tggzzz wrote: ↑Sun Feb 01, 2026 6:20 pm I've already got dotted-LED, VFD, Panaplex, Nixie, Dekatron displays, but haven't got any projection displays - until now.

What's this, no minitron or magic eye? I'm shocked, and worried!

True. No minitrons either, but I'm not going to search for those.

There was one on my father's Heathkit radio tuner, which was used up until it was replaced by a Heathkit AR2000, c1972. The former is long gone, the latter still works (due to my superior soldering, of course!).

Ahh, but do you have any stacked lucite plate displays?
-Pat

Minitrons are not displeasing to look at

Agreed.

But they are too associated with petrol pumps to be of interest to me. Silly, I know.

(Cockpits, their other domain, should only have analogue instruments, preferably not needing electricity)

I have a Numitron timer (only pic I could readily find of it), but have thus far not come across Minitrons.



-Pat

And how about the vertical decimal displays (I've heard them referred to as 'pinball' displays)?
The LSD's decoder has some issues - you can see a few of the neons lighting simultaneously as it counts up...

-Pat