Low pressure gas discharge and particle beams

This apparatus is designed to demonstrate electrical discharges in air at different pressures. It not only demonstrates gas discharge but also can be used to show cathode rays and even be considered as an introduction to particle accelerators.

The complete vacuum tube comprises five parts which can be dismantled to add further versatility to experimenting. The two black collars house the electrodes. They also form seals to the glass tube and bulb ends. The second image shows the centre tube detached from the electrode. The electrodes are metal discs, seen here with a slot cut into it. The 5mm pin on the outside is the electrical connection to the electrode. One electrode also has a tube fitted which needs to be connected to a vacuum pump to remove the air once the tube is assembled.

The two 'bulb' ends have a fluorescent coating on the inside ends.

The following images (D1 to D4) were taken as the air pressure was being reduced by the vacuum pump.

Once the pressure started to fall the air ionised at about 3kV. The distance between the electrodes is 14cm. The negative electrode (Cathode) is on the left of the images and the positive electrode (Anode) is on the right. The positive striated column decreases as the pressure reduces. The colour of the light relates to the spectrum of the gasses in air, mostly Nitrogen.

Eventually the pressure is reduced until only the negative glow can be seen in the tube. As the air is removed further an interesting effect starts. Almost all the visible gas discharge disappears and the tube fluorescent ends start to glow.

The image of the slit can be seen at the anode end of the tube. This is created by electrons (cathode rays) accelerating towards the anode. Some pass through the slit and hit the fluorescent coating.

Behind the cathode end of the tube a similar effect appears but as they are behind the cathode so must be positive rays. There is a glow in this part of the tube and no clear image of the slit in the cathode is formed but the fluorescent coating glows. Interestingly the electron beam at the anode is easily moved with a magnet placed near the tube. The positive rays are not noticeably moved by the magnet. These rays are known as 'Canal rays' and are positive ions created from the remaining air molecules.

Eventually as the last of the air is removed from the tube, the beam current decreases and the voltage needs to be increased to maintain the glowing ends. However, as the applied voltage reaches 10kV X-rays start to be detectable around the anode.

AC gas excitation

The image below shows the gas being excited by a high voltage, high frequency source as in a 'Plasma Globe'. Only one connection is required for this to work. The gas is ionised by the changing alternating field and the fluorescent ends are glowing partly due to the UV content of the light being emitted.

Driven by a plasma globe HT source

Experimental Vacuum tube

Image 1

Electrode with slit assembly Image 2

Discharge starts (D1)

Pressure dropping (D2)

Lower pressure (D3)

Negative glow Almost all the air removed (D4)

With the negative glow just visible the Image of anode slit produce by cathode rays can be seen.

Electron beam

Positive stream of ions at the cathode

Canal rays

The video clip shows the process of air being removed by the vacuum pump. The glowing ends fade away due to the vacuum improving and the voltage reaching a set maximum of 6kV.

Crookes Railway Tube

Crookes tube with 20kV applied

The picture below shows the fluorescent tips on the paddle wheel glowing with electron bombardment as it approaches the anode. Some specs of fluorescents on the anode can also be seen glowing. 'Click on' the picture or icon below to see a short video.

Invented by Sir William Crookes, this evacuated tube has a paddle wheel which can move along the internal glass rails. The paddles are made from mica with fluorescent paint on the tips. A potential difference across the two electrodes can cause electrons to travel from the negative electrode ( Cathode) to the positive electrode (Anode). Since the paddles turn, Crookes suggested that the cathode rays (electrons) must be particles with mass. It was later ascertained that it was not the direct action of electron bombardment that lead to the movement, but the fact that the vacuum is not perfect. The remaining molecules of air act on the paddles either thermally or electrically charged.

Crookes railway tube or paddle wheel circa 1950

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Warning: The vacuum inside this tube is slightly greater than in the 'Canal rays' experiment above. The voltage required to move the paddle wheel is around 20kV therefore X-rays are produced at the anode running this experiment.