Geissler tube with a contact problem

[Physikfan]

To all friends of antique Geissler tubes

The picture shows a complex shaped Geissler tube.


Unfortunately, the corresponding metal cap with eyelet is missing during a contact.
Please, does anyone know how to fix this problem?

Best regards

Physikfan

[jpanhalt]

If there is enough sticking out,just attach to it.   Can't tell from your picture.   If not, how hot does the electrode get?   I'm thinking of a very low-temp alloy or the newer, solderable conductive ink that is pure silver with no organic matrix.

Finally, repairing that one bulb by a half-decent scientific glassblower would be easy, if the glass can be reworked.   Do you know the gas and pressure in the tube?

[Gyro]

That's a beautiful Geissler tube, I haven't seen one like that before.

It would be good if you could post a close-up photo of the damaged end. The internal pressure is very low, so it would collapse during any attempt at glass blowing.

Luckily the current it needs to pass is very low and the voltage required will happily ignore any resistive connection. Your main challenge is to obtain a replacement matching end-cap which is a good match for the other one. It ought to be possible to fabricate something from some thin brass tube or sheet.

For the actual connection, if there is any wire still protruding, I would wrap this with some very fine wire to make the connection and make it secure with a little adhesive. I wouldn't risk soldering that close to the glass, you would possibly damage the seal or crack the glass. You could them form a bundle or pad of wire which will make contact with the replacement end-cap when fitted (or maybe run down the side under the cap). secure the new cap to the glass with something like epoxy glue.

If the lead-out wire is completely gone, then you will have to resort to conductive paint (rear screen repair type) to make the connection.

[jpanhalt]

@Gyro

I think you misunderstood my comment.   Any half-competent glassblower would know to release the vacuum before working on the glass.  Cutting off an electrode like that and replacing it is really quite easy.  Then of course, you need to re-evacuate the device and seal it.

[Physikfan]

The tube is at least over 100 years old.
A glass blower should have available the same kind
of glass for a successful restoration of the tube.

Is there an electrical conductive glue available to make a connection without applying
mechanical stress to the tube?
Also it may not an easy task to find such a new metal
cap.

[jpanhalt]

Here's a link to the conductive ink I mentioned.   It is a variation of the Tollens reagent/reacton: http://pubs.acs.org/doi/abs/10.1021/ja209267c

The reactants are volatile, so reasonably thick conductors of pure silver  are possible.  It is reported to be solderable.

I copied the recipe years ago.  If you want to go in that direction, I can post it here.

EDIT: I have also seen prepared "ink" for sale in small amounts.  Just search for silver ink +the lead author.   That might be more reasonable than making it yourself, if you have limited experience in chemistry.  However, Tollens reaction is one of the first things taught in many laboratory courses.   It is very robust and hard to screw up.   

[Gyro]

@Gyro

I think you misunderstood my comment.   Any half-competent glassblower would know to release the vacuum before working on the glass.  Cutting off an electrode like that and replacing it is really quite easy.  Then of course, you need to re-evacuate the device and seal it.

Sorry, yes of course you are right, I should have said more of what I was thinking. The problem with unsealing the tube is achieving the same pressure and gas composition as the original. Manufacture of these things was, the time, an art rather than a precise science. The it may have been just been a relatively poor vacuum with residual air (mainly nitrogen), or it may have had proportions of various Noble gasses. Nothing so pure as would be achieved today in gas discharge tubes and signage. As the OP says , it is an antique and getting it back to the authentic composition and pressure could be difficult.

[Gyro]

The tube is at least over 100 years old.
A glass blower should have available the same kind
of glass for a successful restoration of the tube.

Is there an electrical conductive glue available to make a connection without applying
mechanical stress to the tube?
Also it may not an easy task to find such a new metal
cap.

I really would still be tempted to try to fabricate a new cap, using some very thin sheet brass (or tube if you can find a matching diameter). Use a wooden mandrel of the same diameter as the glass, shape it and solder the seam together, bend in at the top and do the same and add a wire loop for external connection. It would be a visible repair, but an 'honest' repair. Maybe you can make a conductive 'pad' of scrunched up fine wire to make a light pressure contact between the end of the tube and the cap. The cap could then be glued to the sides of the tube using a ring of compliant rubber type adhesive that would minimise stress on the glass, but could later be removed with solvent if a more 'professional' repair becomes available. Silver loaded epoxy would probably also work, but the result would be a lot more permanent.

[jpanhalt]

I agree, my first approach would be to salvage what is there.   A small spring might come in handy for the contact.   Epoxy has become the duct tape adhesive solution, but consider glyptal (a heavy enamel), which would be more vintage-appropriate as an adhesive. 

[Gyro]

Yes, when I said Silver loaded epoxy would be "a lot more permanent", I should probably have added 'too permanent' compared to a solvent based adhesives.

[oldway]

Spray GRAPHIT 33 where  the metal cap with eyelet was placed, this will make a conductive coating on the glass.
(KONTACT CHEMIE)
https://www.amazon.fr/KONTAKT-CHEMIE-conducteur-graphite-GRAPHIT/dp/B002ZXQY9Q

Protect other places with insulating tape so you will have a conductive coating only where it is needed.

After drying the conductive coating, remove the insulating tape.

Wrap a thin non insulated copper wire on the conductive coating and glue it.

[SeanB]

Most of the caps are cemented on, so a simple cement loaded with graphite powder will make a good enough contact material at the current that these tubes draws. The contact material has to be able to survive high temperature, as the terminals dissipate most of the energy used in the tube.

[KE5FX]

You really only need one connected electrode with a Geissler tube, because the counterpoise is capacitively coupled to the environment anyway.  A couple of centimeters of aquadag or metallic ink should be OK as a contact at the unenergized end, with a meter or so of wire hanging off of it.

[Physikfan]

I hope to have found a usable electrically conductive adhesive now
to solve the problem with the one contact (missing metal cap with eyelet).

[Cyberdragon]

Ahem...allow me to take a quote from the product packaging.

LOW VOLTAGE

Make of that what you will...

[Ian.M]

Also, will it take the heat?

IMHO the best option would be the conductive paint used for car window demister repair to extend the contact from the broken-off pin to the sides of the glass seal, then fabricate a brass cap as Gyro has previously suggested from shim stock, except don't solder the side seam or more than half the end seam, and form it round a smaller diameter mandrel so it grips the sides of the seal by its springyness to make reliable contact with the conductive paint.

[Physikfan]

Unfortunately there are more problems:

Contact with cap:



Contact without cap:

[jpanhalt]

If there is any portion of the electrode sticking though the glass, and I can't imagine a situation where that is not true, why not use a spring to contact the stub.   The current is very small.

You also need to think of whether an action is reversible.   Now you need to deal with removal of the epoxy.  In this case, that should not be too hard.

[Ian.M]

Now we can see the problem better.  A sprung contact mounted on the glass isn't going to work unless its much much bigger than the other cap as there aren't any parallel sides on the seal for it to grip.

I can see why you want to use conductive glue, but for %DEITY%'s sake stay away from Epoxy - use something that can be removed by soaking in an aggressive solvent.

[jpanhalt]

I disagree.  It is a glass-metal seal.  That means there has to be metal to which a spring can make contact.

[Cyberdragon]

Wait, what is going on here?

There looks to be an inner part of the electrode on the broken end, but not on the end with the metal part. However, there looks to be a metal rod in that evacuation tube on that end. Is that part of the other electrode that broke off and fell in there? If that is, it's and you'll need a glassblower to fix it. I would assume the metal rod is not only to provide more surface area for the arc to form, but to keep it away from the seal.

[jpanhalt]

Sorry, it was half-time and I needed to fix dinner.  On further thought...

Do you have a high-voltage, high-frequency source like a Tesla coil, a neon light transformer with a spark gap, or even a diathermy machine?

My thinking is this.   Whatever broke the electrode may have affected the glass-to-metal seal.   The tube may be at atmospheric pressure (or close), and won't work regardless of what you do.

As one responder said earlier, if the tube itself is OK, only a single electrode is needed (hence the Tesla coil test).

If the tube doesn't light with a Tesla coil, the tube is bad.   You need on-site help to fix it.   It is a wonderful piece of history, and I would hate to see it destroyed by someone with no experience.

John

[Neomys Sapiens]

There is a guy in Germany who is refurbishing Geissler tubes and has a lot of info about them. I might did it out but you'll certainly find it when you search for 'Geissler Röhren'.

[Physikfan]

Hi jpanhalt, Gyro, oldway, SeanB, KE5FX, Ian.M, Cyberdragon and Neomys Sapiens

I have followed your suggestions and have provisionally treated the electrode without cap with conductive silver.
The tube is working, the operating voltage is about 13 kV.



Best regards

Physikfan

[Physikfan]

One question remains:
Which gas makes this white discharge?

Regards

Physikfan