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 Ok, so what are they ?
                         Well, essentially, they are glass globes with 'lightning bolts' coming from a central electrode to the surface of the sphere. If you touch the globe, the lightning will normally follow your hand. They were also popular in the eighties! wot's in one ?
    Inside the globe is a mixture of gases (see further below) at a reduced pressure. Less that atmospheric pressure gives longer sparks, or a lower voltage requirement. The typical pressure is 1/10 or 1/20 of an atmosphere. Too dim, and the bolts will be fuzzy. In some cases, the inner bulb will be surrounded by glass, this is the case here. The inner bulb is largely filled with metal, and the space between the small and large bulbs are filled with the inert gases. (See below)

                          There is a high frequency AC, or a high frequency pulsating DC applied to the electrode in the centre. This is in order for the current to get through the glass globe and surrounding air. Most plasma balls work at a few thousand volts, but around 10, 000 volts for 'make your own' ones. Most frequencies are from a few to a few tens of kilohertz.

                          As the plasma is generated, it gets hotter, and since hot air rises, the streamers will tend to move up the sides of the ball. The streamers, as it is plasma, stay intact until an instability breaks them. Thats the principle behind the Jacobs ladder so beloved of horror - film makers..

But how does it work ?

                          It emits light because of  'spontaneous emission'. This basically means that the ion, atom or molecule (the particles) have a higher level of energy than stable ones, so become unstable. The particles emit photons. Through this release, the particles go back to stability, only to start again. Each time a particle goes from the same higher state to lower, the same photon will be emitted. The only way to change the colour of the 'lightning' is to change the gases (see below)

                          The only way a particle becomes excited is if a moving particle comes whizzing past with enough energy. The electron releases energy to the particle by transference. The light we see is by thousands of particles 'spontaneously emitting' at the same time. To achieve the look in plasma balls, we need to have billions of them. A standard 8'' width plasma ball will have approximately 100 billion billion particles. To get an idea, 2500 particles span the length equal to the diameter of  a human hair!!!

                        LIGHTS ON !                             LIGHTS OFF !
   In the plasma ball demonstrated, we believe the inert gases to be neon and argon

    Colours of Gases:
Helium : gives a brilliant white colour
Neon  :  gives red streamers with orange pads at the end (the pads are pinkish red on the pictures above)
Carbon Dioxide : is a white or blue white colour
 Nitrogen :  streamers are usually greyish pink or light orange, depending on the lights and voltage
Oxygen :  and water vapour are basically useless. They don't glow brightly, and require more voltage
Argon :   streamers are violet/lavendar
Argon and neon : have the least voltage requirement. This is what we believe is in the one pictured
 Krypton :  is close to white, or light grey
 Xenon :  streamers are like lightening, and bluish white or bluish grey

(Page contributed by Gareth Alexander , Yr 10 physics student 1999/2000)
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