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Old 29th Apr 2017, 3:30 pm   #1
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Default Huge Crystal Set

Been out to Watford Museum today, another good little museum. The shop was selling the usual stuff except I have never seen large bits of Fools Gold (Iron Pyrites) for sale. I bought one for thirty bob, the rule it is on shows the 6" and 8" marks.

Time to make a huge crystal set for fun!

MM
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Old 29th Apr 2017, 6:32 pm   #2
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Default Re: Huge Crystal Set

Nice one. I also have a chunk of fools gold on the shelf awaiting service in a crystal set, when I have a bit more time. I've been told it can be a frustrating business though, finding a good contact. Are you planning to set it (or a piece of it) in Wood's metal?
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Old 29th Apr 2017, 7:20 pm   #3
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Default Re: Huge Crystal Set

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Originally Posted by PSValves View Post
Are you planning to set it (or a piece of it) in Wood's metal?
That at least would be relatively non-hazardous.

I recall seeing some crystal-sets where the 'lump of Galena' was clamped in a small cup which contained liquid Mercury to provide the electrical contact.
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Old 29th Apr 2017, 8:05 pm   #4
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Default Re: Huge Crystal Set

Quote:
Originally Posted by merlinmaxwell View Post

Time to make a huge crystal set for fun!

MM
Hey Huw,

Wonderful idea! Looking forward to seeing the project unfold!
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Old 29th Apr 2017, 8:36 pm   #5
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Default Re: Huge Crystal Set

Quote:
I recall seeing some crystal-sets where the 'lump of Galena' was clamped in a small cup which contained liquid Mercury to provide the electrical contact.
I have about a litre of mercury but no woods metal, as our cats don't do H&S at all I think a pot of solder will have to do. The other problem is the aerial, all I use now is a Wellbrook loop, I suppose that (at about 50ohm) with a hundred pF in series would emulate a long wire. Cheating a bit I know, plenty of signal! Now where are my Brown Headphones...

Quote:
I've been told it can be a frustrating business though, finding a good contact.
I have plenty of choice.

Quote:
Wonderful idea! Looking forward to seeing the project unfold!
Looks like I am committed to do it, can't disappoint. Time for a skip hunt for wood...
 
Old 29th Apr 2017, 9:21 pm   #6
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Default Re: Huge Crystal Set

Hi Al, have a google for a crystal set with a 1Mtr dia coil. That would be a big set!!

Ed
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Old 29th Apr 2017, 9:24 pm   #7
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Default Re: Huge Crystal Set

My latest crystal set, built a few years ago now, uses a bit of "wild" galena found not far from here. I made the detector optional with two period terminals mounted on the board so I can replace the fiddly home-made galena/spring one with a germanium diode when I get tired of constantly adjusting it to find a spot which works. I also find that even when you have found a good point, it seems to go off after a short while, so it is a continuous fight. Cheating with an OA47 is less trouble. With the Wellbrook antenna, could you not just connect it to the tuning coil of the crystal set via a low tap, or a separate coupling coil kind of variometer style Merlin?
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Old 29th Apr 2017, 9:50 pm   #8
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Default Re: Huge Crystal Set

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Hi Al, have a google for a crystal set with a 1Mtr dia coil. That would be a big set!!
Well, that's the diameter of a Wellbrook loop, right, Ed? Looks intriguing!
I'll get Googling!
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Old 29th Apr 2017, 10:22 pm   #9
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Default Re: Huge Crystal Set

Originally they used 'Woods Metal', an alloy which contains bismuth and melts at low temperature, to mount the crystal in its holder. It's not easy to come by in small quantities and is expensive. (I believe that one of its applications is to mount diamonds when cutting them). However, Carrs make a low melting point solder which melts at well below the boiling point of water - 70 degrees C in fact - hence it's name 'No 70'. You can get it from here:

http://www.chronos.ltd.uk/acatalog/E...__etc_346.html

Hope that's of interest.
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Old 30th Apr 2017, 4:02 am   #10
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Default Re: Huge Crystal Set

How much Wood's metal do you need Merlin? PM me if you like.

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Old 30th Apr 2017, 9:10 am   #11
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Default Re: Huge Crystal Set

Hi Gents, this was not a Wellbrook loop, from memory it was a huge cardboard tube with about 90 turns of thick Litz on it.

Ed
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Old 30th Apr 2017, 9:19 am   #12
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Default Re: Huge Crystal Set

Does anyone know if Wood's metal is the same as old fashioned plumbers lead bars we used to use for wiping lead joints?
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Old 30th Apr 2017, 9:24 am   #13
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Default Re: Huge Crystal Set

As always, Google is your friend!
Look here, for example -

https://en.wikipedia.org/wiki/Wood%27s_metal

Not the same as plumber's solder.
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Old 30th Apr 2017, 1:46 pm   #14
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Default Re: Huge Crystal Set

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Does anyone know if Wood's metal is the same as old fashioned plumbers lead bars we used to use for wiping lead joints?
Definitely not - you need a blowlamp to use 'plumbers' metal' as it was known . If it had melted at such a low temperature joints on hot water pipes would have fallen apart. From memory - and I stand to be corrected - I think that the lower temperature of plumbers' metal is 240 C or thereabouts. The Carrs 'No 70' solder I mentioned in my earlier post (used for soldering 'white metal') has the same melting point as Woods Metal, 70 degrees C, well below the boiling point of water:

https://en.wikipedia.org/wiki/Wood%27s_metal

Wandering off track - waffling and dribbling as old buffers are prone to do - the mention of plumbers' metal brought back memories from days of yore!

As apprentices in the mid 1950s, we were only allowed to use a blowlamp to make 'wiped' joints after we'd mastered the skill of making them with a crucible ('pot') and a ladle, the way that the Romans did it 2000 years ago - splashing molten solder onto the joint with a wooden spatula from the ladle till the joint got hot enough for the solder to adhere to it.

GPO cable jointers who joined 4" lead covered cables made pot joints - very skilled guys having to do that down holes in pavements. Unlike other solders, Plumbers' metal doesn't just go from solid to liquid - it has a temperature range during which it becomes 'plastic'. Get it just a bit too hot and the tin separated from the lead and dripped off the joint, then it's curtains - start all over again.

A lost art - as are 'blown' joints on lead gas pipes made with a meths mouth-lamp - lead is illegal for water and gas nowadays.
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Old 30th Apr 2017, 2:45 pm   #15
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Default Re: Huge Crystal Set

This might be useful:

http://oldheadphones.com/crystal/info/mount.htm
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Old 30th Apr 2017, 3:54 pm   #16
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Default Re: Huge Crystal Set

Nice opportunity to go one better than the twin crystal sets that achieved some popularity back in the day - I'm visualising a switch built on an ebonite panel to connect to any of three or four cat's whiskers mounted around this lump!

Paul
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Old 7th May 2017, 7:26 am   #17
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Default Re: Huge Crystal Set

Here's an article from sept 19, 1923 wireless weekly

I like the bit about mounting the crystal with screws in a cup, then pouring some mercury in around it - but make sure the cup is always upright!!!
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Old 25th May 2017, 8:35 pm   #18
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Default Re: Huge Crystal Set

Interesting video here about gallium - a metal best know to us as an alloy - gallium arsenide, used in semiconductors. The melting point of gallium is approximately 30 C (85F) so it will melt in the palm of a hand. Maybe a bit too low as a substitute for 'Woods Metal'. Other aspects of its behaviour also perhaps make it unsuitable - it expands by 3% on cooling (rather like when water freezes), whereas most metals contract on solidifying. Furthermore, it attacks other metals by diffusing into them at the grain boundaries.

Even so, it's interesting stuff:

https://www.youtube.com/watch?v=HKCKv6OmS0Q

https://en.wikipedia.org/wiki/Gallium

Where can you get it?

Here, at $24 USD for 15 grams:

http://www.mcssl.com/store/solutionm.../gallium-metal

Not really a practical proposition when shipping costs are taken into account:

Quote:

We will procure a quote for your specific city if you send us an email including your city and postal code: info@solutionmaterials.com.
Typical international rates for dangerous goods materials OTHER than Gallium (which is a Class 8 corrosive) are $150-$275 per package.

End quote.

Oh well...back to low melting point solder as a Wood's Metal substitute then.
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Old 26th May 2017, 8:17 am   #19
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Default Re: Huge Crystal Set

It's on hold at the moment, but I have found some ancient conductive epoxy.
 
Old 27th May 2017, 10:56 pm   #20
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Default Re: Huge Crystal Set

I have an old crystal around here somewhere that someone made from a lump of Galena ore and used a 3/8" copper pipe cap to mount the xtal in. I assume they used Woods Metal, but it could have been solder.
When I went to Colorado with my fiance`we stopped at a huge rock shop. I bought many different metallic xtals , maybe a dozen or more, with the intention of using them in radios to see which worked best.
After she was killed, during the moving of her stuff, I put them somewhere. Just can't remember where.
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