Odd electron tube - how do I wire up this physics experiment correctly?

[bjornart]

I also posted this at the Straight Dope, so here's your chance to prove the JREF's a better forum. :D

The school where I work has a lot of different old equipment that no-one currently knows how to use. One such apparatus is this electron tube with helmholtz coils:

Front (http://www.flickr.com/photos/8097032@N02/4273359129/in/set-72157623083585971/)

Back (http://www.flickr.com/photos/8097032@N02/4273359161/in/set-72157623083585971/)

Close-up front (http://www.flickr.com/photos/8097032@N02/4273359071/in/set-72157623083585971/)

Close-up back (http://www.flickr.com/photos/8097032@N02/4273359099/in/set-72157623083585971/)

Sp+ and Sp- are easy, those are the connectors for the coil.

2V and K- are connected to each other and to one of the two connectors on the bottom of the tube.

Uh is connected to one of the 2Ohm connectors, while the other 2Ohm connector is connected to the second connector on the bottom of the tube.

A+ is connected through a 22kOhm resistor to the wire that can be connected to the top of the tube.

As can be seen in the close up, the electrode at the bottom consists of a metal ribbon with a single twist, connecting the two inputs to the tube. There's also a disc connected only on one side, with a hole aligned with the twist in the ribbon.

Rather than experimenting, and possibly frying something irreplaceable (under the current budget regime) I'm hoping someone here has some advice and/or knowledge to share.

[ben m]

Let's see, it looks like some sort of cathode ray experiment. This is complete guesswork, but:

Suppose A+ is the anode and K- is the cathode. If you put the cathode at ground and the anode at +1 kV, you'd be able to accelerate an electron beam from K to A. (Maybe there's some diffuse gas in the tube, like a neon lamp, which lets you see the beam.)

Why would there be a beam at all? Suppose that the "spiral ribbon" is a 2Ohm heating element. Put an ampere or two through it, it heats up, and hot cathodes emit electrons.

The discharge can't run-away and become a spark because of the 22KOhm resistor; if the current gets too high, the voltage sags and it calms back down again. Or at least it doesn't destroy your power supply.

Once you have a beam zipping through the thing, you can turn on the magnetic fields and steer it back and forth.

That leaves one unexplained connector. That one's up to you.

ETA: also, if the rest of my hypothesis is right, I remain completely puzzled at why the cathode would be shaped like it is.

[Perpetual Student]

Is there a way to check the integrity of the vacuum?

[Fnord]

Can you read the part number off the electron tube?

It would look something like "12AU7" (for a dual triode).

My corporate firewall will not allow me to see the images. Are they ... (wait for it) ...









... shocking?

[MattusMaximus]

This looks like a standard setup of Helmholtz coils (http://en.wikipedia.org/wiki/Helmholtz_coil) to me. Such devices are commonly used when performing the usual e/m experiment to determine the ratio of an electron charge to its mass - the two coils have current run around them, generating a magnetic field which turns an electron beam.

I have an almost identical setup at school. If you can wait until tomorrow, I can take a look at the connections to give you a better clue on how to hook it up.

[jasonpatterson]

Such devices are commonly used when performing the usual e/m experiment to determine the ratio of an electron charge to its mass - the two coils have current run around them, generating a magnetic field which turns an electron beam.


This was my thought as well. Once you figure out how to power it, you'd measure the width of the arc created by a given voltage and magnetic field and from that you can figure out e/m. It's also a handy device for showing deflection of charged particles in a magnetic field. The coils will (hopefully) have info printed on them regarding the number of winds of wire inside so you can figure out the magnetic field.

Here's (http://electron9.phys.utk.edu/phys136d/distlab/lab9.htm) some info related to the actual experiment you'd do with the thing. I don't have a classroom right now, so I can't check mine. :( I miss my physics...

[ben m]

This was my thought as well. Once you figure out how to power it, you'd measure the width of the arc created by a given voltage and magnetic field and from that you can figure out e/m. It's also a handy device for showing deflection of charged particles in a magnetic field. The coils will (hopefully) have info printed on them regarding the number of winds of wire inside so you can figure out the magnetic field.

Here's (http://electron9.phys.utk.edu/phys136d/distlab/lab9.htm) some info related to the actual experiment you'd do with the thing. I don't have a classroom right now, so I can't check mine. :( I miss my physics...

Ah, that explains why the cathode is at that funny angle. It's meant to fire the electrons sideways, and you need to use the magnetic field to arc this beam around to the anode. (I was picturing the beam "normally" going straight up, then being bent just a bit to either side by the B- field.) This probably means that my wiring guess was wrong and you probably don't want to put a kilovolt on the anode!

Physics teaching labs have so much more fun equipment than physics research labs.

[bjornart]

See, what puzzles me is that there's no way to power it up, that I can see, that gives me a electron gun at the bottom firing sideways. That's what I thought it would do when I found it.

I could just put 2V AC across the (presumed) cathode connectors and ramp up the cathode-anode voltage until something interesting started to happen, or everything blew up because the near vacuum has been compromised years ago. But I'd like to first figure out what it's supposed to do.

Why for instance do I need to use an external 2 Ohm resistor, which the to green connectors labled 2 Ohm seem to imply?

At some point I thought that since the circular disk will be at a slightly different electrical potential than the center of the twisted ribbon it'd work as an electron gun, but with the miniscule resistance involved (it measures as .3 Ohms, but I suppose it'll increase as it heats up) and the low voltage across it, that doesn't make sense either.

[Lensman]

If you've lost the vacuum, bin the tube, it won't work the way it's supposed to.

[BenBurch]

If you've lost the vacuum, bin the tube, it won't work the way it's supposed to.

Well, the heater will burn out in a flash just like a cracked light bulb. Its pretty obvious.

[bjornart]

Well, the heater will burn out in a flash just like a cracked light bulb. Its pretty obvious.

If that happens I'll have a nifty nicknack for my apartment. :D

[bjornart]

Okay, I've wired it up by educated guesswork. 2V AC across the cathode heater, with a 2 Ohm resistor in series. 1,5-2 kV from cathode to anode gets me a diffuse orange beam, and a drop down to almost no voltage.

24V on the coils though, as indicated for similar devices, does absolutely nothing... that I can observe. Anyone have an idea what voltage would be appropriate?

Each of the coils is labled as having 10 000 windings and a resistance of 4100 ohms. And my ohm-meter agrees.

[MRC_Hans]

Okay, I've wired it up by educated guesswork. 2V AC across the cathode heater, with a 2 Ohm resistor in series. 1,5-2 kV from cathode to anode gets me a diffuse orange beam, and a drop down to almost no voltage.

24V on the coils though, as indicated for similar devices, does absolutely nothing... that I can observe. Anyone have an idea what voltage would be appropriate?

Each of the coils is labled as having 10 000 windings and a resistance of 4100 ohms. And my ohm-meter agrees.The cathode is not an electron gun, just a cathode. The filament should become red-hot. But since you have anode current, your 2V is apparantly adequate.

24V is not much for a 4100ohms open coil; you might want to try some more (monitor their temperature, as the only limit will be heat dissipation). Can you change the polarity of each coil? If they work against each other, you will not see any effect.

Hans

[ben m]

The cathode is not an electron gun, just a cathode. The filament should become red-hot. But since you have anode current, your 2V is apparantly adequate.

24V is not much for a 4100ohms open coil; you might want to try some more (monitor their temperature, as the only limit will be heat dissipation). Can you change the polarity of each coil? If they work against each other, you will not see any effect.

Hans

Yeah, one typically builds such coils at a density such that they run warm. 24V through 4100 Ohms is only 150 milliwatts. That thing looks like it will withstand at least a watt or two---crank it up! 60-100V sounds reasonable. Keep an eye on the coil temperature; warm is fine, "ouch that's hot" is too much.

[MRC_Hans]

Yeah, one typically builds such coils at a density such that they run warm. 24V through 4100 Ohms is only 150 milliwatts. That thing looks like it will withstand at least a watt or two---crank it up! 60-100V sounds reasonable. Keep an eye on the coil temperature; warm is fine, "ouch that's hot" is too much.I'd go for "as long as it doesn't smell ...... much"

This is a demonstration device, it is not built for continuous operation.

Hans

[bjornart]

Woohoo! :D Now it works, although I'm still a bit puzzled as to how.
Getting a decent magnetic field required a bit more voltage, around 200V. And getting a decent beam required a bit more voltage on the filament to get it red-hot. 6V AC did the trick.

With those changes I get electrons going in a nice, vertical circle. If I turn off the magnetic field I get an orange glow around the cathode and anode, and a nice tight beam from the cathode, moving pretty much straight out of the hole in the disc, horizontally, at 90 degrees to what I'd presume was the strongest part of the electric field. It's pretty much a completely straight line between filament and glass, with a nice blue spot on the glass.

Could someone explain that to me?

Also, what health and safety warnings would the aparatus have come with? The nocebo effect is giving me tingling in fingers and a slight headache.:o

[Cuddles]

Also, what health and safety warnings would the aparatus have come with? The nocebo effect is giving me tingling in fingers and a slight headache.:o

Do not tumble dry.

Seriously though, aside from the obvious high voltage, temperature and vacuum warnings, there really shouldn't be any risk involved. It will be an incredibly low current, low energy electron beam so there's no risk from radiation, and there's no way to actually get hit by the beam since you'd have to break the vacuum to do so.

[Jack by the hedge]

Woohoo! :D Now it works... I get electrons going in a nice, vertical circle. If I turn off the magnetic field I get an orange glow around the cathode and anode, and a nice tight beam from the cathode, moving pretty much straight out of the hole in the disc, horizontally, at 90 degrees to what I'd presume was the strongest part of the electric field. It's pretty much a completely straight line between filament and glass, with a nice blue spot on the glass.

That's very pleasing. Any chance of some pictures of it in action?

[MattusMaximus]

Woohoo! :D Now it works, although I'm still a bit puzzled as to how.
Getting a decent magnetic field required a bit more voltage, around 200V. And getting a decent beam required a bit more voltage on the filament to get it red-hot. 6V AC did the trick.

Sorry that I never got back to you, I got busy grading final exams :boggled:

If I recall correctly about my setup, it also required roughly 6 volts AC to get the heating filament up to the level where it emitted a necessary number of electrons to make the beam visible.

As for the 200V, are you sure you aren't talking about the voltage which generates the electric field? That seems to agree with what my Helmholtz coils use: about 200-300V DC to accelerate the electrons away from the cathode and generate the straight beam in the first place.

To generate a magnetic field capable of effectively turning the electron beam, my coils need to have about 2-3 Amps DC passing through them. I cannot recall exactly what voltage this requires, but if you've got 24V pushing current through 4100 Ohms of resistance, this seems to generate too little current for the needed magnetic field.

With those changes I get electrons going in a nice, vertical circle. If I turn off the magnetic field I get an orange glow around the cathode and anode, and a nice tight beam from the cathode, moving pretty much straight out of the hole in the disc, horizontally, at 90 degrees to what I'd presume was the strongest part of the electric magnetic field. It's pretty much a completely straight line between filament and glass, with a nice blue spot on the glass.

Could someone explain that to me?

I can't be certain without seeing a picture, but I think what you're seeing when the magnetic field is off is simply what I described above. The cathode filament heats up, and the 200V DC creates an electric field which goes from the anode to the cathode, accelerating the electrons from the cathode to the anode in a straight line. This is what generates the straight beam you see. The beam won't bend until you push some current through the coils and generate a magnetic field passing through the middle of them perpendicular to the motion of the electrons. I assume you are familiar with the Lorentz force (http://en.wikipedia.org/wiki/Lorentz_force), correct?

Also, what health and safety warnings would the aparatus have come with? The nocebo effect is giving me tingling in fingers and a slight headache.:o

What Cuddles said. So long as you don't do anything stupid with the high-voltage stuff, like drop water on it while it's running, or drop the entire apparatus on your foot from the lab bench, I don't think you've got any worries. Electrons that are accelerated via 200-300V are pretty low energy and won't be emitting any nasty radiation when they hit a target. If it's Bremsstrahlung radiation (http://en.wikipedia.org/wiki/Braking_radiation) that you're worried about, you can't really start emitting dangerous x-rays from electrons slamming a target until you get up into the many, many thousands of volts range.

[bjornart]

I've taken a few pictures, but it's not the easiest sight to capture with a digital compact. You can click through the set to see the electrons bent by the magnetic field, but this first one (http://www.flickr.com/photos/8097032@N02/4287205115/) shows what puzzles me. You can see the glow coming from the anode at the top of the tube, and the electron beam coming horizontally from the filament through the hole in the disk part of the cathode, at 90 degrees to the straight line between cathode and anode.

It's obviously accelerated by the cathode-anode voltage, since there's no beam without it, but how this works puzzles me.

After your advice and my experimentation it's currently wired up like this:

Coils 2 x 10000 windings, 8200 Ohms total, 180-200V

Heating voltage, filament in series with 2 Ohm resistor, 6,3V AC

Cathode-anode voltage 200-350 V DC. 200V gives a tighter and more focused circle. 350V gives the largest possible circle, but also causes most of the gass in the bulb to glow.

And just so there's no doubt about why the direction of the beam confuses me, the diagram below shows the anode as a, the cathode as H (heater) and d (holed disk), and the beam as the horisontal dotted line---.
a
, - ~ a ~ - ,
, ' aaa ' ,
, ,
, ,
, ,
, ,
, ,
, d ,
,------------H ,
, dH ,
, H , '
' - , __H__ , '

[jasonpatterson]

Deleted for stupidity and failure to learn basic reading skills.

EDIT:
OK, I'm trying to figure this out for your coil, but I can't see the setup of the tube well enough in your picture for it to make sense. Normally the cathode is surrounded by a cage of sorts that causes the electrons to shoot off of the filament. Some of these wind up going through a gap in the cage (the hole in your disk) and forming the beam. It's basically a little particle accelerator... How is the disk electrically connected to the rest of the thing? Is it part of the cathode? In any case, I found an operating manual (ftp://ftp.pasco.com/Support/Documents/English/SE/SE-9638/012-03471d.pdf) for a similar instrument from PASCO that includes some explanation and a decent diagram of how the electron gun portion of the device works.

[jasonpatterson]

OK, does the filament actually sit inside the hole in the disk, or even protrude through it a bit? If so, any electrons released by that portion of the filament are being shoved away from the disk (which is part of the high voltage cathode) and making the beam. Since it's a disk with a hole in the middle, locally the highest potential the electrons can get to is along the central axis of the disk (they'd have to go through a low potential region to cross the disk's face) so they get shot sideways rather than directly at the anode.

If the filament rests behind the disk completely, I'm stumped from what I can see, unless the disk carries a high potential somehow...

[bjornart]

OK, does the filament actually sit inside the hole in the disk, or even protrude through it a bit? If so, any electrons released by that portion of the filament are being shoved away from the disk (which is part of the high voltage cathode) and making the beam. Since it's a disk with a hole in the middle, locally the highest potential the electrons can get to is along the central axis of the disk (they'd have to go through a low potential region to cross the disk's face) so they get shot sideways rather than directly at the anode.

If the filament rests behind the disk completely, I'm stumped from what I can see, unless the disk carries a high potential somehow...

If it protrudes, it's not by much, but that's the best (only) explanation so far, and it makes sense. I'll have a closer look tomorrow.

[kalen]

It seems to me the filament is also the cathode. The disk is the anode of the electron gun.

Electrons are accelerated through the potential between the filament and disk. Some of the electrons end up going through the hole. On the other side of the hole, the electric field is (close to) zero, so the electrons just merrily sail along until they hit the wall.

Of course, the tube has a low pressure gas in it. Some of the electrons excite the gas atoms which give off photons when they return to the ground state. This way you can see the path of the electrons, and how they curve when you put some current through the Helmholtz coils.

[brantc]

That is a gyroradius, or Lorentz force demonstrator.

I know how the Lorentz force works from generators, wiring electromagnets, using my magnetometer. In our lab we have have cold cathode inverted magnetron and ionization gauges for vacuum but how does the Lorentz Force really work?

What is it about the magnetic field that produces a force that is 90 degrees on an electron through a B field(around or cutting the vector)?

What quantum or physical principle?

[bjornart]

It seems to me the filament is also the cathode. The disk is the anode of the electron gun.

Electrons are accelerated through the potential between the filament and disk. Some of the electrons end up going through the hole. On the other side of the hole, the electric field is (close to) zero, so the electrons just merrily sail along until they hit the wall.

That would be an explanation except:

The disk is connected to one end of the filament, and the filament has very low resistance, so the electric field between them is minute.
The filament and the disk is connected to a low AC voltage, so the the electric field is not only minute, it switches directions 50 times a second.
There is no beam without the 200-350V anode-cathode voltage.
See?

[bjornart]

OK, does the filament actually sit inside the hole in the disk, or even protrude through it a bit? If so, any electrons released by that portion of the filament are being shoved away from the disk (which is part of the high voltage cathode) and making the beam. Since it's a disk with a hole in the middle, locally the highest potential the electrons can get to is along the central axis of the disk (they'd have to go through a low potential region to cross the disk's face) so they get shot sideways rather than directly at the anode.

If the filament rests behind the disk completely, I'm stumped from what I can see, unless the disk carries a high potential somehow...

The filament actually rests almost a millimeter behind the disk. It's actually flattened and indented right behind the hole.

[Cuddles]

What is it about the magnetic field that produces a force that is 90 degrees on an electron through a B field(around or cutting the vector)?

What quantum or physical principle?

Maxwell's laws in the classical view, QED in the quantum view. If you want an answer why those things work the way they do, that's not an answer you're ever going to get. There will always be more whys to ask in science, and it's either turtles all the way down or there's a point where we simply can't answer the next why. At the moment, this is that point.

[Jack by the hedge]

The filament actually rests almost a millimeter behind the disk. It's actually flattened and indented right behind the hole.

Indented - from which side? Do you mean the ribbon is embossed with a peak pointing towards the hole?

If so, the concentration of excess charge approaching a sharp peak would tend to make emission start there (rather like a sharply pointed lightning conductor). The emitted electrons would spray off in the direction of the hole. The disc is probably only there to focus the beam then, not to make the beam form in the first place.

[jasonpatterson]

Can you see where the high voltage cathode is, as a separate object, or is it part of the hot cathode/filament thingy?

Crazy idea of the day:
The 6V AC makes the filament heat up, producing the free electrons for the beam. The entire cathode, coil, disk, and all is at -2kV with respect to the anode, with very low current flowing through the filament (not enough to heat it or anything, by a long shot, and I believe you said there was a 22kOhm resistor on the HV anode, though a tenth of an amp is still a lot... Your HV power supply is current limited though.) That would make the cathode's voltage oscillate within a few volts of -2kV, and any free electrons simply have to make it off the filament in the direction of the hole to form the beam. Any that do are accelerated by the cathode in a beam to the side for the reason mentioned previously. Since both the disk and filament are at roughly the same potential at any time, there's not much of anything to inhibit the electron's flight through the disk, and the little dent on the center of the coil is concentrating electrons and helping the beam to form in the right spot (so you don't get a beam shooting in both directions.)

[bjornart]

Indented - from which side? Do you mean the ribbon is embossed with a peak pointing towards the hole?

The other way. There's a dimple when seen from the side the disk's on.

Can you see where the high voltage cathode is, as a separate object, or is it part of the hot cathode/filament thingy?

If you read the first post, and look at the close up of the cathode/filament thingy (http://www.flickr.com/photos/8097032@N02/4273359099/in/set-72157623083585971/), you'll see that there are only two wires coming into the bulb at the bottom. Cathode voltage and one side of the heating voltage is connected to one of them.

Does someone have an application for showing field strength around arbitrarily shaped electrodes?

[Jack by the hedge]

The other way. There's a dimple when seen from the side the disk's on.

Ah. Well, I'm back to being confused, then. I have no idea what makes that spray electrons through the hole in the disc rather than any other direction.

[bjornart]

Just in case someone digs this up and wants an answer:

I finally figured out, with a little help, that around a disk shaped cathode the electrical field will be perpendicular to the disk, so thermal electrons wandering through the hole will be accelerated in the manner observed. Looking at older cathode ray tube experiments online may of them have the anode off on a branch, despite being made to observe electrons moving through the tube in straight lines.