Mirror as ground plane

At what frequency? Your problem will be mainly making a good physical contact with the mirror. Aluminium has pretty good conductivity but an annoying oxide coat. Gold sputtered mirrors would be easier to solder.

Indium is used for ground planes that you can see though. Radio astronomers tend to use glass with fine copper mesh embedded in it to keep the control room electronics from drowning out natural signals.

None.

It's difficult to get much below 100ohm per square with metal layers. You can't get thinner than one atom thick, and while in theory the microcrystals of metal can offer a circuitious path, you are laying them down at random.

I made the mistake of asking for vacuum deposited layer of carbon with highish resistance, and got told off by the people who did it.

We ended up using a thick film layer of some very resistive metal oxide-based thick-film ink - about a micron of it.

The best quality second-surface mirrors are coated with silver, followed by copper plating and a coat of paint. Those should be pretty good if you can get wires on them.

Poorer ones have aluminum coatings around 2-3 nm thick and no plating.

Cheers

Phil Hobbs

Ron Wagner used ~dish soap once, to get high resistivity films on some electrostatic panels.

You can imagine trying to get predictable or durable perforance that way . . . .

RL

I think pressure contact will be good enough, so long as its renewed regularly, maybe with a conductive grit.

Will have to treat the isolated glass surface with something to reduce static, or stuff will start flying away at inconvenient times.

RL

A classic way to connect to such things is conductive silver epoxy, probably to silver-plated copper wire. Not tin-plated for long-term use.

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As for shielding effectiveness, the key question is the resistance of a square of the coating, connected only on opposite parallel sides.

Probably won't work, between high square resistance and oxide layer preventing reliable connection.

Though people do use metallized Mylar film, with a long bare tinned copper ground wire in direct contact with the aluminum side, and in a cable this does work.

Joe Gwinn

Won’t work on an unplated film, of course. Indium solder applied with an ultrasonic iron is one approach—you need to avoid cracking the film, which will cause extreme flakiness.

Cheers

Phil Hobbs

At high enough frequencies there is no need for direct connection. Adhesive copper tape stuck to the varnish film on the back of an ordinary mirror may provide enough capacitive coupling to make a good connection. It all depends on the proposed application, which we don't know. John

If you use a conductive colloidal copper electrical joint compound such as Thomas & Betts Kopr-Shield or Burndy Penetrox E between your copper plated surface and a copper alloy spring contact the need to renew should be eliminated.

This stuff is essential for reliable mechanical Cu-Cu connections in a corrosive environment.

Agreed,

Joe

Why use glass? Why not copperclad FR4?

Rugged repeated use - subjected to fire and impact.

Will be possibly burning FR4 in contact with it.

A work surface.

RL

This can even work at highish audio frequencies:

I was trying to trace some disconnected house wiring by feeding about

The signal led towards an outer wall of the house which had had a garage built onto it. From inside the house, the wires appeared to be running along the wall in the garage , but there were no wires visible in the garage ... and from the garage, the signal appeared to be coming from inside the house.

Then I realised that there was a large mirror inside the house on that wall and the signal was being capacitively coupled to the top edge of the mirror by wires that must have been at least a foot above it and separated by a plasterboard [drywall] ceiling. The whole mirror was re-radiating the signal.

Stainless steel is pretty conductive.

It's interesting to walk around and listen to ambient e and h fields, and light too.

You can trace wires by listening to the 50/60 Hz fields, usually with lots of harmonics.

I once built an IR detector into the body of a little hand torch so that I could carry it unobtrusively around the site where I worked. The management hadn't told us they were installing surveillance equipment, but the IR illuminators for the hidden cameras showed up quite clearly.

Yes, if they are the only wires you are interested in; across open ground, for instance. In a house, when trying to trace a specific wire, you need some sort of identifying signal such as a bleeper. I also have an exceptionally noisy electric drill that can be used to mark a circuit (it was used in the construction of the Brabazon airliner and pre-dates effective suppression methods).

It is easiest to track the magnetic field if you can get to both ends of the wire and make a complete circuit, but if the wire has been cut and you don't know where it goes, the electrostatic field is more traceable as long as there isn't a lot of earthed metal or damp material in the way. Sometimes there is enough capacitance to earth at the cut end of the cable that some highish frequency current can be driven down it and make electromagnetic detection possible.

It wouldn't be hard to include a circuit that down-samples ultrasonics.

But it's covered in a thin coherent layer of chromium oxide, which is pretty conductive, but to a degree that depends on the pH of the monolayer of water adsorbed on the surface. Some of my colleagues once wanted to use it for the electrodes in a conductivity meter and it messed up the accuracy for high-conductivity water (2% NaOH in the worst case - not exactly water as you want to meet it).

which glass ain't . . .

Mind you, they're both just peachy to bend, drill, or to work with simple hand tools - luckily avoided here once sharp edges are tamed.

The consideration only came up when the basic material showed up, free, begging for safe 'disposal'.

Not sure if the idea might be useful to others, elsewhere.

RL