boiling off electrons

[RichD]

I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes? Is this like synchronized diving,
they're trained to hit the water at specified spots?

--
Rich

 

In sci.physics.electromag RichD <r_delaney2001@yahoo.com> wrote:

I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

To put it simply, the cathode material is chosen to be a material
that releases electrons when heated.

Google thermionic emission if you want details.

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes? Is this like synchronized diving,
they're trained to hit the water at specified spots?

They do sometimes but they mostly don't as the grid is negative and
serves to throttle the electron flow.

Google control grid if you want details.

--
Jim Pennino

Remove .spam.sux to reply.

 

[Phil Allison]

"RichD"

I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil?

** It's analogous to a liquid boiling and releasing molecules of itself as
vapour.

Are there degrees of boiling, like a pot of water,

** Yeah - like there's " old boiler " & " pot boiler " ......

or is there an on/off threshold?

** Strictly temperature related, just like ordinary boiling.

Only it needs to be in a vacuum to be effective.

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that?

** Cos it is 99% open space.

Look at a pic of a grid for god's sake.

How do they find the holes?

** Sigh......

Is this like synchronized diving,

** Yeah - they do clever back flips and all.

Wanker.


...... Phil


> they're trained to hit the water at specified spots?

 

[Jon Kirwan]

On Thu, 22 Jan 2009 06:15:01 GMT, jimp@specsol.spam.sux.com wrote:

In sci.physics.electromag RichD <r_delaney2001@yahoo.com> wrote:
I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

To put it simply, the cathode material is chosen to be a material
that releases electrons when heated.

Google thermionic emission if you want details.
snip

+ Richardson equation; work function; partial field emission; LaB6;
tungsten; pure field emission (hot [schottky] or cold [very tiny
tip]); quantum tunneling; Fowler/Nordheim curves; Wehnelt.

Jon

 

[jmfbahciv]

Jon Kirwan wrote:

On Thu, 22 Jan 2009 06:15:01 GMT, jimp@specsol.spam.sux.com wrote:

In sci.physics.electromag RichD <r_delaney2001@yahoo.com> wrote:
I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?
To put it simply, the cathode material is chosen to be a material
that releases electrons when heated.

Google thermionic emission if you want details.
snip

+ Richardson equation; work function; partial field emission; LaB6;
tungsten; pure field emission (hot [schottky] or cold [very tiny
tip]); quantum tunneling; Fowler/Nordheim curves; Wehnelt.

Jon

Oh, man! I've only seen two of the items on your list in my
lifetime.

/BAH

 

[Jon Kirwan]

On Thu, 22 Jan 2009 06:41:38 -0500, jmfbahciv <jmfbahciv@aol> wrote:

Jon Kirwan wrote:
On Thu, 22 Jan 2009 06:15:01 GMT, jimp@specsol.spam.sux.com wrote:

In sci.physics.electromag RichD <r_delaney2001@yahoo.com> wrote:
I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?
To put it simply, the cathode material is chosen to be a material
that releases electrons when heated.

Google thermionic emission if you want details.
snip

+ Richardson equation; work function; partial field emission; LaB6;
tungsten; pure field emission (hot [schottky] or cold [very tiny
tip]); quantum tunneling; Fowler/Nordheim curves; Wehnelt.

Jon

Oh, man! I've only seen two of the items on your list in my
lifetime.

I worked for a short time (a year +) with _the_ electron microscope
company in a small technical capacity. Some things managed to rub
off. The emitters I worked with used a combination of thermionic
emission and field emission (partial) and the Wehnelt is pretty much
impossible to miss, as well. LaB6 was one of those materials
sputtered on for enhanced effect, as I recall, on some emitters (other
materials were also used.) Work function is obvious -- can't avoid
the concept for even a few seconds, as it permeates everything
interesting in the world. Richardson, Fowler, and Nordheim get name
dropped from time to time, as do others.

Jon

 

[Salmon Egg]

In article
<630c2b4c-5174-4ba9-b53d-0ad1977b7426@i24g2000prf.googlegroups.com>,
RichD <r_delaney2001@yahoo.com> wrote:

I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes? Is this like synchronized diving,
they're trained to hit the water at specified spots?

--
Rich

Look at: http://en.wikipedia.org/wiki/Thermionic_emission

In other responses red herrings like anodes and grids were mentioned.
While important for devices, such elements have little to do with the
fundamental process at the electron emitter.


Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

 

[Benj]

On Jan 22, 12:57 am, RichD <r_delaney2...@yahoo.com> wrote:

I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

Not quite a correct description, because once they "boil off" it is
the electric fields in the tube that cause them to be accelerated and
"fly" to the anode.

How does an electron boil? Can anyone explain this?

Pretty much like water does. In water you have a "sea" of water
molecules bouncing around off each other. The amount of action if
determined by the temperature of the water. And the distribution of
water molecule velocities has a statistical distribution with some
having high velocities and some low. When you start to have lots of
high velocity molecules some of them start to leave the liquid and are
vapor (or a cloud of individual molecules).

In a metal, one pretty much has a "sea" of free electrons. These also
have a statistical distribution of velocities that depends on
temperature. Again, if you get the temperature high enough there are
enough "fast" electrons that they are leaving the metal.

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

There is not a distinct 'boiling point" like with water, but the
number of electrons escaping does depend on temperature. Usually this
temperature has to be pretty high. Edison discovered the tube by
noting the current that flowed when he put a metal plate inside one of
his light bulbs. However there are distinct lifetime advantages if
you can get eletron emission at lower temperatures. 20th century tube
"technology" was very much involved in the black magic of producing
long-lived low temperature high current cathodes. Today, some of this
old technology appears to be lost, like the technology of how to make
a decent mummy.

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes? Is this like synchronized diving,
they're trained to hit the water at specified spots?

A grid is just a bunch of wires that is use to create an electric
field to accelerate the electrons away from the cathode. (or repel
them back to it). Electrons DO smash into it. But like a garden hose
at chicken wire, most of them go through the holes. The grid is MOSTLY
"holes"! There is no need to have them all go through the holes.
[Well usually, if we were to discuss the theory of beam power tubes,
we'd talk about the problems of grid heating due to the electrons that
smash in, but this is no problem in smaller tubes.] PLUS, it depends
on how the tube is being operated. If the grid is negative, then it is
creating a repelling field. The field gets stronger as you get near
any given grid wire so the electrons tend to be deflected away from
the grid wires. OK?

 

[Benj]

On Jan 22, 6:33 am, Jon Kirwan <j...@infinitefactors.org> wrote:

+ Richardson equation; work function; partial field emission; LaB6;
tungsten; pure field emission (hot [schottky] or cold [very tiny
tip]); quantum tunneling; Fowler/Nordheim curves; Wehnelt.

Oh look Muffy! It's a guy who knows "everything" but is willing to
explain nothing!
(much of which has little to do with the subject in question except
tangentially)

Electron microscopes are not "tubes".

 

[Jon Kirwan]

On Thu, 22 Jan 2009 10:27:50 -0800 (PST), Benj <bjacoby@iwaynet.net>
wrote:

On Jan 22, 6:33 am, Jon Kirwan <j...@infinitefactors.org> wrote:

+ Richardson equation; work function; partial field emission; LaB6;
tungsten; pure field emission (hot [schottky] or cold [very tiny
tip]); quantum tunneling; Fowler/Nordheim curves; Wehnelt.

Oh look Muffy! It's a guy who knows "everything" but is willing to
explain nothing! (much of which has little to do with the subject
in question except tangentially)

I honestly don't have the time to carefully walk someone from 0 to 60,
right now. But there is plenty on the web to read.

Electron microscopes are not "tubes".

No, but that doesn't mean that the terms I provided are not relevant
to a search, does it?

Jon

 

[Rich Grise]

On Thu, 22 Jan 2009 10:27:50 -0800, Benj wrote:

On Jan 22, 6:33 am, Jon Kirwan <j...@infinitefactors.org> wrote:

+ Richardson equation; work function; partial field emission; LaB6;
tungsten; pure field emission (hot [schottky] or cold [very tiny
tip]); quantum tunneling; Fowler/Nordheim curves; Wehnelt.

Oh look Muffy! It's a guy who knows "everything" but is willing to
explain nothing!
(much of which has little to do with the subject in question except
tangentially)

Electron microscopes are not "tubes".

They certainly are! They're just significantly larger than, say, a 4-400A,
and the target (DUT) is the anode. ;-)

Hope This Helps!
Rich

 

[BradGuth]

On Jan 21, 9:57 pm, RichD <r_delaney2...@yahoo.com> wrote:

I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil?  Can anyone explain this?

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that?  How do
they find the holes?  Is this like synchronized diving,
they're trained to hit the water at specified spots?

--
Rich

Thorium gives off electrons rather nicely when heated, although radium
(Ra226) gives off electrons when stone cold or even when cryogenic
cold. Radium is a cold cathode electron emitter, and otherwise
extremely nifty beyond most imaginations. Radium is also one of the
most secretly horded elements on Earth, with a 1600+ year half life to
boot.

~ BG

 

[Salmon Egg]

In article
<2edcf569-c6ce-476b-bb6b-d6ce2eec05e5@p2g2000prn.googlegroups.com>,
BradGuth <bradguth@gmail.com> wrote:

Thorium gives off electrons rather nicely when heated, although radium
(Ra226) gives off electrons when stone cold or even when cryogenic
cold. Radium is a cold cathode electron emitter, and otherwise
extremely nifty beyond most imaginations. Radium is also one of the
most secretly horded elements on Earth, with a 1600+ year half life to
boot.

Radium is an ALPHA emitter, not an electron emitter. It is possible that
some daughter isotopes are BETA (electron) emitters. Radium is in the
same column of the periodic chart as is barium and strontium. These are
sometimes used, especially as oxides, for low temperature cathodes,
Thus, radium is also likely to be a relatively low temperature
thermionic emitter, stupid as it may be to use it as such,

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

 

[hhc314@yahoo.com]

On Jan 22, 8:45 pm, Salmon Egg <Salmon...@sbcglobal.net> wrote:

In article
2edcf569-c6ce-476b-bb6b-d6ce2eec0...@p2g2000prn.googlegroups.com>,

 BradGuth <bradg...@gmail.com> wrote:
Thorium gives off electrons rather nicely when heated, although radium
(Ra226) gives off electrons when stone cold or even when cryogenic
cold.  Radium is a cold cathode electron emitter, and otherwise
extremely nifty beyond most imaginations.  Radium is also one of the
most secretly horded elements on Earth, with a 1600+ year half life to
boot.

Radium is an ALPHA emitter, not an electron emitter. It is possible that
some daughter isotopes are BETA (electron) emitters. Radium is in the
same column of the periodic chart as is barium and strontium. These are
sometimes used, especially as oxides, for low temperature cathodes,
Thus, radium is also likely to be a relatively low temperature
thermionic emitter, stupid as it may be to use it as such,

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

ROFL Bill,

This thread is getting more humorous as it proceeds. Let me add my bit
to the chuckle heap.

Radium indeed emits Alpha particles naturally. Still, when heated to
near incandescent termperature, is capable of emitting electrons. Not
strange, very little information is published on Radium's surface
barrier potential for electron emission.

Perhaps our govenment will find a way to fund such "Important"
research. :-<

Harry C.

 

[Benj]

On Jan 22, 8:45 pm, Salmon Egg <Salmon...@sbcglobal.net> wrote:

In article
2edcf569-c6ce-476b-bb6b-d6ce2eec0...@p2g2000prn.googlegroups.com>,

BradGuth <bradg...@gmail.com> wrote:
Thorium gives off electrons rather nicely when heated, although radium
(Ra226) gives off electrons when stone cold or even when cryogenic
cold. Radium is a cold cathode electron emitter, and otherwise
extremely nifty beyond most imaginations. Radium is also one of the
most secretly horded elements on Earth, with a 1600+ year half life to
boot.

Radium is an ALPHA emitter, not an electron emitter. It is possible that
some daughter isotopes are BETA (electron) emitters. Radium is in the
same column of the periodic chart as is barium and strontium. These are
sometimes used, especially as oxides, for low temperature cathodes,
Thus, radium is also likely to be a relatively low temperature
thermionic emitter, stupid as it may be to use it as such,

This could explain a factoid I know. I do know that right before
transistors put tubes mostly out of business, tube research was making
great strides. I was assured that a cold cathode had been developed
that produced enough current to power a vidicon tube. I was told that
developers were quite confident the current levels could be easily
increased to more typical tube service with some development. (As you
know the big shortcoming of tubes was the energy wasted heating the
cathodes). But all that technology seems to have disappeared along
with the big tube manufacturers of the day. Today even the standard
tube technology seems to have largely evaporated leaving former
communist countries and third world countries to make some less than
stellar tubes to satisfy tube-loving guitar players.

It is in interesting story that I once applied for a job at MIT and
based on the above information I told the interviewer that I thought
transistors were a "passing fad". I didn't get the job! Later my
mistake became obvious. Do you know what it was? In those days both
tubes and transistors were constructed as 3 dimensional devices. Hence
with no heater losses they were basically equivalent devices with the
tubes having MUCH superior performance in the day. But it was the
Fairchild "planar" process that made the "integrated circuit" possible
that eventually made solid state the hands-down winner over 3-D wired
and spot welded construction. I doubt that the MIT interviewer knew
this at the time, but doubtless thought I was "wrong" based on the
large power needed to run tube heaters. It's hell knowing too
much!

 

[Salmon Egg]

In article
<563cf945-51ec-4669-898b-488dafeffd3e@e1g2000pra.googlegroups.com>,
Benj <bjacoby@iwaynet.net> wrote:

This could explain a factoid I know. I do know that right before
transistors put tubes mostly out of business, tube research was making
great strides. I was assured that a cold cathode had been developed
that produced enough current to power a vidicon tube. I was told that
developers were quite confident the current levels could be easily
increased to more typical tube service with some development. (As you
know the big shortcoming of tubes was the energy wasted heating the
cathodes). But all that technology seems to have disappeared along
with the big tube manufacturers of the day. Today even the standard
tube technology seems to have largely evaporated leaving former
communist countries and third world countries to make some less than
stellar tubes to satisfy tube-loving guitar players.

I had posted that the extra elements in a vacuum had little effect on
the basic thermionic emission processes. While basically true, some
tubes with field emission cathodes have been produced. In particular,
x-ray tubes requiring relatively little current at high voltage fit that
bill. A high voltage on a nearby anode can produce significant current
from field emission points without preheating the cathode.

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

 

[Frnak McKenney]

On Fri, 23 Jan 2009 22:49:21 -0800, Salmon Egg <SalmonEgg@sbcglobal.net> wrote:

In article
563cf945-51ec-4669-898b-488dafeffd3e@e1g2000pra.googlegroups.com>,
Benj <bjacoby@iwaynet.net> wrote:

This could explain a factoid I know. I do know that right before
transistors put tubes mostly out of business, tube research was making
great strides. I was assured that a cold cathode had been developed
that produced enough current to power a vidicon tube.
--snip--
I had posted that the extra elements in a vacuum had little effect on
the basic thermionic emission processes. While basically true, some
tubes with field emission cathodes have been produced. In particular,
x-ray tubes requiring relatively little current at high voltage fit that
bill. A high voltage on a nearby anode can produce significant current
from field emission points without preheating the cathode.

A few years back there was a great deal of interest in array-cathode
CRTs called Field Emitter Displays. The basic design involved what
amounted (my view) to two slightly separated plates of glass; the
"front" plate was phosphor-coated, much like the usual CRT face, but
in place of the usual point-source electron gun whose electron stream
had to be bent, twisted, and hosed across the entire face, the FEDs'
back plate was a (dense) XY grid of emission points. Each phosphor dot
was illuminated by multiple cathode points; this redundancy would have
allowed a much larger manufacturing yield than single-point-failure LCD
displays, and much larger screens.

It sounded like a good idea, but I haven't heard FEDs mentioned lately;
I don't know if they ran into technical problems or just got hit with
the "LCDs are close enough" end of the VHS-Beta stick.


Frank McKenney
--
Fashion is...a search for a new language to discredit the old,
a way in which each generation can repudiate its immediate
predecessor and distinguish itself from it.
-- Fernand Braudel/Civilization & Capitalism, 15th-18th Century
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

 

[Eeyore]

RichD wrote:

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that?

Some of them DO ! Look for example at limits for screen current.

Graham

 

[Eeyore]

BradGuth wrote:

On Jan 21, 9:57 pm, RichD <r_delaney2...@yahoo.com> wrote:
I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes? Is this like synchronized diving,
they're trained to hit the water at specified spots?

Thorium gives off electrons rather nicely when heated

And thoriated filaments / heaters were found to work rather well around
the time of WW2 luckily for the radar pioneers.

Graham

 

[John Larkin]

On Thu, 22 Jan 2009 10:19:05 -0800 (PST), Benj <bjacoby@iwaynet.net>
wrote:

On Jan 22, 12:57 am, RichD <r_delaney2...@yahoo.com> wrote:
I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

Not quite a correct description, because once they "boil off" it is
the electric fields in the tube that cause them to be accelerated and
"fly" to the anode.

Not exactly. Electrons will flow from cathode to anode with zero
field, or even a bit of reverse field. The electrons ate kicked out of
the cathode with a couple ev of energy. There have been thermionic
generators that work that way.

Positive field helps a lot, of course.

John