Radio Tuning The Quicksilver Way

Modern radios are often digital affairs, in which the frequency is derived from a stable crystal oscillator and varied through a microprocessor controlled frequency synthesiser. It won’t drift, and it’s exactly on the frequency dialed in. Older radios though relied on a tuned circuit, a combination of capacitor and inductor, for their frequency selection. If you were curious enough to peer inside — and we know you were — you’d have seen the moving vanes of a variable capacitor controlled by the tuning knob.

Of course, there is another way to adjust a tuned circuit: by changing the value of the inductor. Older car radios for instance moved a ferrite slug inside a coil to tune from station to station. But that method is not good enough for [David Mills]. Being in possession of some finely graduated syringes he decided to try liquid tuning by increasing the volume within the coil.

Solutions of salts made little difference, so he reached for some mercury. The result is an RF inductor wound round a syringe body, with a body of mercury whose position can be adjusted by the plunger. He measures the Q factor of the coil with air core or mercury core, and as the inductance decreases with more mercury, so does the Q.

We see home-made parts from time to time, and there’s nothing too special about permeability tuning. However, this unusual take on the matter makes this one rather special. We doubt we’ll see its like very often in the future.

58 thoughts on “Radio Tuning The Quicksilver Way

    1. I like the idea of a tunable antenna made out of a liquid metal (ideally one that does not wet its containment). Mercury is probably not the best idea from a health point of view though and Gallium is less than idea because wets just about everything except for quartz, graphite, and Teflon.

      I really like the idea of an antenna that could tune from say the 15 meter band to the 23 cm band.

    2. Mercury?
      Loadsa things you can do with it:

      1 – mix with solder to make low melting-point solder. Good for thermal fuses for transformers. Even make a metal that melts at body temperature (spoons that melt in coffee from plaster mould ;-)

      2 – Put in a syringe with 50% oil. Glue alu foil 1/2 way round tube. Keep horisontal and use as tuning capacitor when rotated. You can even glue the syringe to the tuning knob :-) Not very good for mobile use, but perfect for bench use.

      3 – It used to be used in church clocks as a vertical column on the pendulum. Temp coefficient different to iron pendulum, so can cancel out the expansion of the iron – CofG remains the same and clock not temperature dependent.

      3 – Use expansion properties in capiliary tube to make designer temperature switches.

  1. Tuning a coil with a slug of metal has been used before, not with mercury but with a little lump of brass. I think it was mainly used in high frequency circuits where the early ferrite and iron powder magnetic materials were far too lossy.

    1. it’s still used when the core has cracked and can’t be removed. the tech breaks up some ferrite and throws it into the transformer until reaching the required inductance. to go the other way brass shavings are added. then everything is sealed with wax.

  2. This stuff is nasty! Evaporates at room temperature. 4 digit hazmat team cleanup bills from just breaking a mercury thermometer, and hazmat IS what it takes to clean it up. A single day in a room with it takes your IQ straight down a couple points and never lets it back up. Like chewing lead paint chips on steroids. Fun stuff sure! Neat. Kewl. Can hack with it. Let me set off your wallet alarm system…. am sure that still works even if grey matter doesn’t anymore due to poisoning. HAZMAT teams bill you at least 4 digits plus decimal and two more digits for cleanup of this stuff if you get caught and OMG if you report it yourself you are so screwed, typical way to get saved… errr… caught…. errr… saved! Billed. And for gosh sake, never call the fire department cause you dropped a compact fluorescent bulb and it broke, just open windows and go to motel for a week. (Unbelievable but true, the bill crippled families some yrs ago).

    Not an alarmist. Watched it happen more than once. Only other thing worse would be an article about magic radium paint you just dip the brush, lick it and twist to a point, then paint the digits on the clock REAL careful and precise. Story ends with exhumed later and encased as hazardous radioactive waste, no marker, not sure which state you’re in. We kewl with that right?

    You would understand if I were talking about Radium. Why ppl get dumb with anything less… is… well… likely due to previous contamination! Plain and simple. Call Mom and ask about Mercury. But hey… they solved that radium problem, dug those ladies up and encased them in steel drums and buried them in the desert. We’re safe! You’re safe… who we talking about again? Wasn’t this about rabbits or squirrels or something…. I fergit.

    You have no scope of the idea here. It is accumulative. You can do nothing to reliably reduce it. You accumulate it.

    You have ONE protection. Avoidance.

      1. Seriously this fool went off worse than I did I while back about HF. It’s not like he’s playing around with mercury salts or something…it’s elemental mercury and yeah basic safety measures are more than sufficient.

        Lol Hazmat clean up for a broken fluorescent lamp…give me a break

        1. The problem is, occupational exposure limits for a lot of stuff is way way lower than acceptable for incidental occasional exposure, as they should be really, 25 years of dealing with the stuff daily will leave a mark. That’s why a hazmat team will be all over top moon suited, they have to deal with the shit daily.

          However, you get the occupational safety trained crowd going off like a pinch of salt will kill you, because it’s toxic, whereas you need a pretty large salt shaker full. Now it’s useful to be all anal like this in a work setting, but it does seem a bit cry wolf-ish to anyone who might use a substance a couple of times a year in the shop at home.

          1. Thing is, OHS is **Occupational** Health and Safety. If you do this at home (or anything else dangerous at home), and you don’t work from home, then you don’t have to worry!

          2. I smell what you’re stepping in…i work in a machine shop and deal with my fair share of nasty chemicals, even ended up in the ER from Naphtha exposure (turns out I’m allergic to it).

            Toxic? Yes. Dangerous? Possibly. Nervous breakdown over exposure? Probably not.

    1. David – the author here.
      It’s nowhere near that bad – Hg has a pretty low vapour pressure, so you have to wait a long time for enough to get into bulk air to cause problems. That said good luck to you if you take a sniff from the top of a bottle of mercury.

      Small spillage can be cleaned up with some sulphur powder. The Hg will bind with the sulphur and eventually react to form a solid sulphide.

      Don’t do silly things with Hg (I’d probably count my experiment as silly) and you’ll be ok.

      Don’t make methyl mercury.

    2. Forget the mercury and radium. You should be warning people about the dangers of this deadly substances.
      In fact, I think just a few weeks ago as many as 80 people in Houston were reported to have been killed by it. Really scary stuff! Start spreading the word to raise awareness and save lives. ;)

  3. This hacking, using mercury… it’s a GREAT example of using a liquid metal and I applaud it as a great proof of concept hack that it really is. But it is hazardous. Our user here taught. But this isn’t one to replicate, we have his data so can avoid the hazard.

    1. When I went to the dentist in the 1960’s I’d get a drop or two of mercury to play with just for the asking. If it reduced my IQ I must have been born smarter than Einstein.

      Occasional minor exposure is not the same as chronic exposure. Yes, the government regulations have completely gotten out of hand. But the actual risks have not changed. The advent of parts per trillion measurements has given ignorant journalists lots of material to get eyeballs.

      1. Yah… they gotcha with that amalgam. A lifetime of leaching into your system.

        It’s interesting to see folks defending mercury and lead paint chips after so much evidence has been amassed.

  4. Yeah, talk about wetting. I want to say mercury can diffuse through plastics and really should be in glass. Maybe can diffuse through some glasses too. Not sure on the later. Used to be used for mercury diffusion pumps to make electronic components I read in “200 Meter on Down” and uhm… I have to read more what is posted on here. Accelerator stuff. I had an idea using a glass syringe and something like a screw to tune the plunger… and viola… the reference permeability tuned oscillator is doing about the same. I wonder how carbon, mixtures of carbon and unique ferrite mixtures would perform. This looks like something I might try as I was thinking about making some ferrite and grinding some “D cell” carbon rods for a range of absorptive or reflective paints.

      1. Yes, most do not blow glass for their vacuum tubes or have particle accelerators. I forget… I used gloves when I worked with the ICP-AE diffusion pumps. Fun with maintenance! Messy stuff when gets on you, eh? I was around aluminum all day with remnants of lead and cleaned for extra hours when I did that job. :-|) Amazing what the alchemist and tanneries went through. To think Newton, Dr. Benjamin Rush and other were exposed to regularly or even prescribed mercury.

        Amazing fact is the Lewis and Clark Expedition has been verified based on the mercury tracing.

    1. As long as you set aside enough money for the one who has to clean up the mess.
      Of course you have the liberty to poison yourself. But not the others. And not to make someone else pay for it.

  5. Is it some sort of stupid joke that half of everyone posting here today is paralyzingly scared of a little mercury? Ridiculous. And here I thought that most HAD readers were smart enough to know how to safely handle heavy metals, but not those particular posters apparently. I suppose that HAD will have to supply warnings any time they write articles that include heavy metals. That means anything involving soldering, mercury, and even computers and many other normal household objects.

    1. No kidding. Where I come from there are plenty of other alibis for why people recreationally poison people. I don’t get it? Flint water, the water, Tulip farms, Yeah, I guess you didn’t grow up between Detroit and Chicago cancer belt, uh, huh??? Oh, we’ll just keep the Nuclear Reactors running with criminals and EMF hack everyone to death like the sonic assaults in Cuba seems the norm around the Great Lakes with scapegoats and all like the Little Caesars commercials, shut down the FCC Stations and have the Coast Guard and other assigned to detect EMF not do their job…

      and you can stick it in your mouth, though I can’t find that commercial… it’s the “Where to we want it choir.” :-|). .

      Where’d they put it?

      1. That would be an awesome experiment to try. Perhaps capacitively couple to the thermometer/antenna and use a UHF antenna analyzer to determine resonant frequency, and then come up with a conversion algorithm to convert that information to temperature? Seems tricky, but might just be possible.

  6. I didn’t like the fact that the syringe is open. Push it to far and you have mercury squirting out. But… air has to be able to go somewhere or it cannot move. How can that be solved?

    I thought about putting a tiny balloon on the end. That might create back pressure that would keep pushing the plunger back out. What else might work?

    How about attaching a second syringe to the end of the first?!?! That way if you push too far the mercury just goes into the second syringe. Now the mercury is more or less contained in a closed system.

  7. Excellent observation on the fact that the syringe is open.

    You an take advantage of the size and materials of the second syringe (say with spring backing on the plunger) or balloon k constant to calculate a force that would be needed to counter act. Other than inductive coupling (unless maybe shielded well (e.g. copper, muMetal, Steel, etc.) and/or maybe at a distance less shielded) an electromagnet on the tuner plunger could be made to tune the syringe very accurately. Thinking the shielding is most important if using an electromagnet to tune.

    I like the idea the gentlemen said above also noting using mixtures of ferrite, barium ferrite, and brass. What would carbon do to the mix?

    Would be interesting to see what NaK tunes in with the same apparatus.

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