Exploring Cathode Rays & Anodes: A Visual Guide

[pkc111]

TL;DR Summary

Im trying to visualise the shape of the field lines in the crookes tube apparatus used with the maltese cross placed after the anode.

The attached diagram shows the cathode rays moving in straight lines forward but the anode is beneath them?

 

[Baluncore]

The cross, anode and screen end of the tube was near ground potential while the cathode was held at a high negative voltage. Once the electrons accelerate away from the cathode they continue to radiate in their initial direction since they are surrounded by ground potential. The image appeared as fluorescence of contaminants in the glass.

 

[pkc111]

So it would have worked the same without an anode?

 

[Baluncore]

So it would have worked the same without an anode?

No. The glass wall is a good insulator. Without an anode on the inside of the evacuated glass envelope, it would have been hard to get a significant cathode current flowing.

The inside of the screen end of a modern CRT is coated in colloidal graphite called “aquadag”. The anode is in contact with that conductive surface on the inside of the tube. The aquadag functions as an electrostatic Faraday screen.
https://en.wikipedia.org/wiki/Aquadag

 

[pkc111]

OK. So what are the electric field line patterns in the Maltese Cross Crookes Tube?

 

[sophiecentaur]

So it would have worked the same without an anode?

You should not pay too much attention to that diagram. Imo, it's more of an artist's impression than a working device. How is the Maltese cross connected? If it is floating then the negative charge would just build up until all the arriving electrons would be deflected away and not form a clear shadow. In the diagram, as it stands, the electrons from the cathode would move more or less straight towards the anode. In the electron tubes I have seen (in all their different forms) the anode has the form of a ring which, without any other modifications, would pull the electrons towards it. Some of them would go straight down the middle and past the ring because of the screening effect of nearby electrons. They would follow the same sort of orbit as a comet around the Sun and some would reach the target. In a large enough tube, they would return, via a long route, to the ring anode. In a working electron tube, an 'electron gun' structure is used with a negatively charged focus ring to keep the electrons in the middle of the beam.
Go hunting for some other electron tube diagrams and you will see what I mean.

 

[Baluncore]

The builder of a Crookes tube would have done a number of things to get a good result. The cross and the anode were certainly both connected to ground, and I think we can assume that the internal surface of the tube was partly conductive.

The emission of electrons from the cathode was due to the impact of positive ions from the gas remaining in the partially evacuated envelope. The cathode is mounted at the thin end of the tube and almost certainly has a radial electric field. You can imply the electric field lines because they are the lines from the cathode to the screen that provide the shadow of the cross. However the gradient of the field throughout the grounded anode end of the tube is low, so the electrons appear not to deviate from their earlier radial trajectory while casting the shadow.

For some reason, Crookes tubes have traditionally used the Maltese cross. The choice of the symmetrical Maltese cross with it's connection to St John, certainly makes for interesting semiotics. However, as an anode, the eight pointed star could have been chosen because it might release the sought-after positive charge carrier atoms from the points, which might explain why the cathode at the time was operated cold. The shadow of the cross, seen in the florescence of the glass, might not have been expected, but it would have been a happy surprise.

 

[pkc111]

OK so here is a photo of a real one. Are the field lines curved down toward the anode?

 

[Baluncore]

Are the field lines curved down toward the anode?

"Down"? gravity is irrelevant, the electrons travel from a negative to a more positive potential. The Crookes tube was an accident waiting to happen. It was not designed by a specialist in electron optics. Where are you trying to go with this thread?

The cathode-rays = electrons, causing florescence of the glass bulb surrounding the cross, must be reaching the anode terminal via the contaminated internal surface of the tube. The entire bulb and cross outside the cathode neck becomes the "anode". The internal coating on the glass will not be visible.

We can see that the tube was only partially evacuated by the plasma forming in the narrow necks near the electrodes. We know that plasma from near the anode sprays the inside of the envelope with anode metal when the tube is first powered.

Maybe the manufacturer got best results by flaming the inside of the bulb before sealing. That could provide the network of surface carbon needed to conduct electrons to the anode terminal via the wall.

Plasma in the cathode neck would have ionised and stripped the surface there, making a better glass insulator. I expect a tube like that always works better after it has been “warmed up”, or they would have thrown it away.

 

[pkc111]

Thanks. I am trying to apply my understanding of charges experiences forces from interacting with electric field lines, to what's happening in the Crooke's tube here.
I said down beacuse that was the relative location of the anode to the cathode in the diagram, nothing to do with gravity.

 

[sophiecentaur]

OK so here is a photo of a real one. Are the field lines curved down toward the anode?

Looking at the green glow on that tube face, it seems as if the lower part is brighter than the upper part. That could suggest (reasonably?) that the fields due to the anode and internal connection are causing the fields around the cross to 'bend' downwards. The drawing is clearly over-simplified.
The tube is much simpler than the sort of thing I was originally commenting on (see this link for a school demo model) but there must be a reason for using an anode in addition to the cross. I guess it still manages to produce a stronger accelerating field for the emitted electrons (about twice - due to half the distance). After passing the anode, the electrons are more in 'free fall', the field component between the anode and cross will be zero with a downwards tilt. Mutual repulsion of the electrons will continue to diverge the beam. This is a crude version of the arrangement shown in my link which uses a circular anode which, at the very least, gives some symmetry. So called electron optics is a very subtle technique and has advanced greatly over the years. Your version is groundbreaking in its way, of course.